Best Heartworm Medicine for Dogs: Vet-Ranked Preventives and What Each Costs Per Month

The best heartworm medicine for dogs is the preventive your vet prescribes, started after a negative heartworm test in dogs 7 months or older and given on schedule all year. Every approved product here works when it is given on time. What separates them is the parasites covered beyond heartworm, the safety profile, the form, and the cost per month.

For most dogs, Heartgard Plus is the best overall pick: a monthly ivermectin chew labeled from 6 weeks of age that also treats hookworms and roundworms, at the lowest listed cost per month. Dogs with tapeworm, flea, or tick exposure, or dogs that refuse pills, suit one of the other six.

Prices are Chewy listings as of October 13, 2026, converted to a cost per month for the weight band shown. Sentinel Spectrum had no listed price available to compare, so check the current listing for it.

How to choose a heartworm medicine for your dog

All FDA-approved heartworm preventives work when they are given on time, every time, as the American Heartworm Society (AHS) puts it. So the choice is not about raw heartworm protection. It is about what else your dog needs covered, which safety cautions apply, which form you can give reliably, and the cost per month.

Every option also starts the same way: a prescription from your vet and, for dogs 7 months or older, a negative heartworm test. For most dogs that need heartworm protection plus coverage of common intestinal worms, Heartgard Plus is the top pick.

The AHS canine guidelines say prescribing a heartworm preventive requires a licensed veterinarian with a valid relationship with you and your dog. If there are no records of past testing and treatment, the dog has to be tested before any preventive is dispensed or prescribed.

Testing rules depend on age:

• Puppies should start a macrocyclic lactone preventive as early as possible and no later than 8 weeks of age, per the AHS. The Merck Veterinary Manual says no pretest is needed for puppies starting at 6 to 8 weeks. Our guide to heartworm prevention for puppies covers that first year.
• Dogs 7 months or older starting prevention need antigen and microfilaria testing first, according to the AHS, and Merck recommends another negative test 6 to 7 months later.
• Every dog over 7 months needs an antigen and microfilaria test each year, even on year-round prevention, because annual testing confirms the program is working.

The test matters because Merck warns that hypersensitivity reactions are a risk after macrocyclic lactone therapy in dogs with microfilaria. A heartworm test for dogs also has an early blind spot: the AHS says antigen and microfilariae are first detectable about 5 and 6 months after infection.

That delay follows the parasite's biology. Heartworm, Dirofilaria immitis, spreads during mosquito season, and VCA Animal Hospitals says the worms take five to seven months to mature. For the full life cycle, from mosquito bite to adult worm, see how do dogs get heartworm.

Once mature, the worms are long-lived: VCA says adults may live five to seven years, while Merck's owner page gives three to five, so the sources differ. VCA adds that the disease is rare in dogs under a year old. Prevention runs year-round, for life, for four reasons:

• Every state: the AHS says heartworm disease has been diagnosed in dogs in every US state.
• Unpredictable timing: Merck says transmission timing is hard to predict, so year-round use of a macrocyclic lactone, continued for the rest of the dog's life, is the only way to reliably prevent infection.
• Resistance: the AHS and FDA recommend year-round dosing, and the AHS says it matters more given documented resistant heartworm subpopulations.
• An early start: Ohio State's veterinary college says prevention should begin at the first puppy exam and continue for life, year-round or through your local mosquito season.

For the wider prevention picture, read heartworm in dogs: how to prevent this silent killer.

Which product suits which dog comes down to a few common situations: herding breeds that may carry the MDR1 gene mutation, dogs with a seizure history, pill refusers, and dogs exposed to fleas, ticks, or tapeworms. Weigh four factors:

• Parasites covered: some preventives target heartworm and intestinal worms only, while others add fleas, ticks, or mites. The AHS says your vet knows which parasites are common where you live.
• Safety cautions: the flea and tick combinations built on isoxazolines carry seizure cautions, while macrocyclic lactones at labeled doses are considered safe even in MDR1 dogs.
• Form: a monthly chew or tablet, a monthly topical, or a vet-given injection every 6 or 12 months.
• Cost per month: the box price divided by the months it covers, at your dog's weight band.

Our picks: the best heartworm medicines for dogs

These seven are ranked on parasites covered, label safety cautions, form, and listed price. Every heartworm preventative here needs a prescription, whether you order heartworm meds online or collect the medication at the clinic. Each box is a weight-band dosage, so match the band to your dog's current weight instead of using a dosage calculator.

Heartworm medication works by treating larvae from the past month or longer, per the AHS, and none of these is a heartworm cure. Treatment is a separate, vet-run protocol: AHS guidelines rule out preventive doses alone, and the Merck Veterinary Manual says clearing infection typically requires melarsomine, an arsenical adulticide.

Merck adds that chronic use of certain macrocyclic lactones with doxycycline, the approach owners search as moxi-doxy, has also been effective, and the AHS describes months of exercise restriction during treatment. For Diroban, survival rate and life expectancy after treatment, and how late is too late, see heartworm treatment for dogs. Common label questions:

• Flea and tick brands: check whether a brand such as K9 Advantix II or Frontline Plus lists heartworm disease among the parasites it covers; if not, it is not a heartworm preventive.
• NexGard: afoxolaner on its own is an isoxazoline, which Merck says should not be a stand-alone heartworm program. NexGard PLUS adds moxidectin for heartworm.
• Store-shelf dewormers: if a dewormer, a Purina allwormer product included, is sold without a prescription, it is not a heartworm preventive, since the AHS says every heartworm preventive is prescription-only.

Heartgard Plus: best overall heartworm-only chew

From ChewyIn stock

Heartgard Plus Chew for Dogs, 26-50 lbs, (Green Box), 12 Chews (12-mos. supply)

A monthly ivermectin and pyrantel chew for heartworm, hookworms, and roundworms, labeled from 6 weeks of age.

$135.69

4.9

Buy on Chewy

Webvet may earn a commission when you click through to Chewy, at no extra cost to you.

Heartgard Plus pairs ivermectin with pyrantel in a monthly chew that prevents heartworm disease and treats and controls hookworms and roundworms, according to Boehringer Ingelheim. The maker says it is safe for puppies 6 weeks or older with no weight restriction.

The tradeoff is scope, with no flea or tick coverage, and the maker reports digestive and neurologic side effects and itching after use.

It suits most dogs, including collies, shepherds, and other herding breeds at the labeled dose: Ohio State says standard preventive doses of all macrocyclic lactones are safe in all breeds. The 26-50 lb 12-month supply listed at $135.69, about $11.31 per month. More in our Heartgard Plus for dogs guide.

Interceptor Plus: best for dogs that also need tapeworm coverage

Vet Rx

From Chewy

Interceptor Plus Chew for Dogs, 50.1-100 lbs, 6 Chews (6-month supply)

A monthly milbemycin oxime and praziquantel chew that adds whipworm and tapeworm control to heartworm prevention.

$91.10

Buy on Chewy

Webvet may earn a commission when you click through to Chewy, at no extra cost to you.

Interceptor Plus is a monthly milbemycin oxime and praziquantel chew that prevents heartworm and treats and controls adult hookworm, roundworm, whipworm, and tapeworm infections, per Elanco. It is labeled for dogs and puppies 6 weeks and older that weigh at least 2 pounds.

Like Heartgard Plus, it has no flea or tick coverage, and the listed price applies to the 50.1-100 lb band only.

It suits dogs at risk of tapeworms or whipworms, and its 2-pound minimum makes it workable for small dogs in the right band. The 50.1-100 lb six-month box listed at $91.10, about $15.18 per month. More in our Interceptor Plus for dogs guide.

Sentinel Spectrum: best for adding flea egg control

From ChewyIn stock

Sentinel Spectrum Chew for Dogs, 25.1-50 lbs, (Yellow Box), 6 Chews (6-mos. supply)

A monthly chew pairing heartworm prevention with lufenuron flea population control and tapeworm coverage.

Check current price →

Buy on Chewy

Webvet may earn a commission when you click through to Chewy, at no extra cost to you.

Sentinel Spectrum combines milbemycin oxime, lufenuron, and praziquantel in a monthly chew that Merck Animal Health says protects against 6 different parasites, including tapeworms. It is indicated for heartworm prevention and flea population control.

It lists no tick claim, and the label cautions that fewer than 6 monthly doses after the last mosquito exposure may not give complete heartworm prevention, one more reason to give it year-round.

It suits homes where fleas keep returning but ticks are rare and tapeworms are a concern. No listed Chewy price was available for the 25.1-50 lb six-chew box on October 13, 2026.

Trifexis: best all-in-one for flea and heartworm without ticks

From ChewyIn stock

Trifexis Chewable Tablet for Dogs, 10.1-20 lbs (6 Chewable Tablets)

A monthly beef-flavored tablet that kills fleas, prevents heartworm, and treats three intestinal worms.

$162.19

4.8

Buy on Chewy

Webvet may earn a commission when you click through to Chewy, at no extra cost to you.

Trifexis pairs spinosad with milbemycin oxime in a monthly beef-flavored tablet that kills fleas and prevents flea infestations, prevents heartworm disease, and treats and controls adult hookworm, roundworm, and whipworm, per Elanco. It comes in 5 doses covering dogs from 5 to 120 lbs.

It has no tick claim, and the label says to use it with caution in dogs with pre-existing epilepsy.

It suits dogs with a real flea problem and little tick exposure that will eat a flavored tablet. The 10.1-20 lb six-tablet box listed at $162.19, about $27.03 per month in that band.

Simparica Trio: best flea, tick, and heartworm combination

From ChewyIn stock

Simparica Trio Chewable Tablet for Dogs, 22.1-44.0 lbs, 3 Chewable Tablets (3-mos. supply)

One monthly chew for heartworm, fleas, six types of ticks, and intestinal worms, for dogs 8 weeks and older.

$119.61

Buy on Chewy

Webvet may earn a commission when you click through to Chewy, at no extra cost to you.

The FDA approved Simparica Trio (sarolaner, moxidectin, and pyrantel) as a once-a-month chewable for dogs and puppies 8 weeks and older weighing 2.8 pounds or more. Zoetis says it was the first chew to protect against heartworm disease, 6 types of ticks, fleas, and intestinal worms in one monthly chew.

Sarolaner is an isoxazoline, and the label says to use Simparica Trio with caution in dogs with a history of seizures.

It suits dogs with regular tick and flea exposure. The 22.1-44 lb three-chew box listed at $119.61 on October 13, 2026, about $39.87 per month.

NexGard Plus: best for dogs already on NexGard

From ChewyIn stock

NexGard PLUS Chewables for Dogs, 17.1-33 lbs. (Gold Box) 3 Chewable Tablets (3-mos. supply)

A monthly chew combining afoxolaner with moxidectin for heartworm, for puppies 8 weeks and 4 pounds or more.

$122.89

Buy on Chewy

Webvet may earn a commission when you click through to Chewy, at no extra cost to you.

NexGard PLUS combines afoxolaner, moxidectin, and pyrantel in a monthly chew. Afoxolaner is the isoxazoline flea and tick ingredient, and moxidectin supplies the heartworm prevention. Boehringer Ingelheim says the chews are safe for puppies 8 weeks or older weighing 4 pounds or more.

Its label calls for caution in dogs with a history of seizures or neurologic disorders, and dogs must test negative for heartworm before starting.

It suits dogs already doing well on a NexGard product whose vet wants heartworm coverage in the same chew. The 17.1-33 lb three-chew box listed at $122.89 on October 13, 2026, about $40.96 per month. Compare the two in Simparica Trio vs NexGard Plus.

Revolution: best topical for dogs that refuse pills

From ChewyIn stock

Revolution Topical Solution for Dogs, 40.1-85 lbs (3 Doses)

A monthly selamectin topical applied at the base of the neck, covering heartworm, fleas, and ear mites.

$106.88

4.7

Buy on Chewy

Webvet may earn a commission when you click through to Chewy, at no extra cost to you.

Revolution is a monthly selamectin topical that Zoetis says protects dogs from heartworm disease, fleas, and ear mites. It is also used on dogs against the American dog tick and canine sarcoptic mange, and it is labeled for dogs 6 weeks and older.

Its tick coverage is limited to the American dog tick, and at about $35.63 per month it costs more than any heartworm-only pick.

It suits dogs that spit out chews or refuse pills, since nothing has to be swallowed. The 40.1-85 lb three-dose box listed at $106.88, about $35.63 per month.

Heartworm-only chews vs flea, tick, and heartworm combinations

Heartworm-only chews versus combination products is really a question about which other parasites your dog faces. The AHS describes three tiers: preventives that only prevent heartworms, those that add intestinal parasites, and broad products that also cover fleas, ticks, and mites. Here is what each combination adds over a heartworm-only chew:

• Sentinel Spectrum: flea population control with lufenuron, plus tapeworms; no tick claim.
• Trifexis: kills fleas with spinosad and covers three intestinal worms; no tick claim.
• Simparica Trio: fleas, 6 types of ticks, and intestinal worms, with sarolaner as the flea and tick drug.
• NexGard Plus: fleas and ticks through afoxolaner.
• Revolution: fleas, ear mites, the American dog tick, and sarcoptic mange.

A heartworm-only chew still does more than heartworm: the Merck Veterinary Manual notes many macrocyclic lactone preventives also prevent other nematode parasites when given continuously. A 3 in 1 flea, tick, and heartworm chew makes sense when ticks are a real local risk.

Does Bravecto prevent heartworm? Look for heartworm disease on the label. Merck says isoxazoline flea and tick drugs such as fluralaner, afoxolaner, and sarolaner should not be used as a stand-alone heartworm program, though they can add a second line of defense. Simparica Trio and NexGard Plus get their heartworm prevention from moxidectin.

For more flea, tick, and heartworm options, see our guide to flea tick heartworm prevention for dogs.

Choosing between a heartworm-only chew and a combination

Among the combinations with a listed price, Trifexis ran about $27.03 per month in the 10.1-20 lb band, Revolution about $35.63 in the 40.1-85 lb band, Simparica Trio about $39.87 in the 22.1-44 lb band, and NexGard Plus about $40.96 in the 17.1-33 lb band. Use these rules of thumb:

• Stay heartworm-only when your dog has no flea or tick problem, has a seizure history, or already uses a flea and tick product your vet is happy with.
• Move to a combination when fleas or ticks are a regular local problem and one monthly chew is easier than two products.
• Compare costs fairly: a combination's cost per month replaces both the heartworm chew and a separate flea and tick product.

Which is safer, Heartgard or Simparica?

No primary source compares Heartgard Plus and Simparica Trio head to head on safety, so the answer comes from their labels. Heartgard Plus is ivermectin and pyrantel; its maker reports digestive and neurologic side effects and itching. Simparica Trio adds sarolaner, an isoxazoline that brings a seizure-history caution, and Heartgard Plus contains no isoxazoline.

Why do some vets not recommend Simparica Trio?

The hesitation centers on its isoxazoline component. The FDA says isoxazolines have been associated with muscle tremors, ataxia, and seizures in some dogs, even without a prior history, though most dogs have no neurologic reactions. The FDA still considers the class safe and effective and advises reviewing each dog's history with the vet.

What dogs should not take Simparica Trio?

Based on the FDA approval and the Zoetis label, these dogs need a different product or a careful vet decision:

• Puppies younger than 8 weeks, or dogs under 2.8 pounds, which fall outside the approval.
• Dogs with a history of seizures, unless your vet judges the benefit worth the labeled caution.
• Any dog without a prescription, or a dog 7 months or older without a negative heartworm test.

What each heartworm medicine costs per month

Cost per month for each product by weight band is the fairest comparison, because every box is priced for one weight band and a set number of months. These figures divide each product's listed Chewy price on October 13, 2026 by the months in the box:

• Heartgard Plus Chew for Dogs, 26-50 lb, 12 chews: $135.69, about $11.31 per month.
• Interceptor Plus Chew for Dogs, 50.1-100 lb, 6 chews: $91.10, about $15.18 per month.
• Trifexis, 10.1-20 lb, 6 tablets: $162.19, about $27.03 per month.
• Revolution, 40.1-85 lb, 3 doses: $106.88, about $35.63 per month.
• Simparica Trio, 22.1-44 lb, 3 chews: $119.61, about $39.87 per month.
• NexGard Plus, 17.1-33 lb, 3 chews: $122.89, about $40.96 per month.
• Sentinel Spectrum: no listed price was available to compare.

For a 40-pound dog, two priced boxes fit: Heartgard Plus 26-50 lb at about $11.31 per month and Simparica Trio 22.1-44 lb at about $39.87, which also covers fleas and ticks. Revolution's priced band starts at 40.1 lb, and the Sentinel Spectrum 25.1-50 lb box had no listed price. Each band is priced separately, so compare the box for your dog's weight.

For the injection, Zoetis says ProHeart cost varies with your dog's weight and claims a ProHeart 12 shot costs about the same, on average, as 12 months of a monthly preventive. No published per-month figure exists for it, so ask your clinic for a quote.

Prevention is the cheap side of the ledger. The AHS calls preventives relatively inexpensive, while treating an infection is costly and needs multiple vet visits and months of exercise restriction. For that comparison, see heartworm treatment cost for dogs.

How much does a 6-month supply of Simparica Trio cost?

On October 13, 2026, Chewy listed the 22.1-44 lb three-chew box of Simparica Trio at $119.61, so a 6-month supply in that band is two boxes, about $239.22, or about $39.87 per month. Other weight bands are priced separately, and Zoetis directs one chew a month.

Compare that with Heartgard Plus at about $11.31 per month plus whatever separate flea and tick product your dog would otherwise need.

The safest heartworm medicine: ivermectin, milbemycin, and moxidectin explained

Ivermectin, milbemycin oxime, moxidectin, and selamectin all belong to one drug class, the macrocyclic lactones. The AHS says these drugs, given according to label instructions, are highly effective and among the safest medications used in veterinary medicine.

Merck says all of them are highly effective at killing the L3 and early L4 heartworm stages. A 2024 review in Frontiers in Veterinary Science states they are the only drug class licensed for heartworm prophylaxis; that review was funded by Zoetis and written by Zoetis employees. Here is which ingredient each product uses:

• Ivermectin: Heartgard Plus.
• Milbemycin oxime: Interceptor Plus, Sentinel Spectrum, and Trifexis.
• Moxidectin: Simparica Trio, NexGard Plus, and the ProHeart injection.
• Selamectin: Revolution.

Owners who want heartworm prevention without ivermectin can choose any of the other three ingredients. At labeled doses, though, that is a preference rather than a safety requirement, even for herding breeds.

Ivermectin vs milbemycin vs moxidectin in herding breeds

Ohio State says some collies and other breeds with the MDR1 gene mutation (also called ABCB1) are highly sensitive to P-glycoprotein-inhibiting drugs. Even so, Merck says all macrocyclic lactone preventives are considered safe in MDR1 dogs at approved dosages, and Ohio State says standard preventive doses are safe in all breeds.

The trouble starts outside the label. The AHS says toxicities in these dogs have been reported with overdosing or with other P-glycoprotein-inhibiting drugs, and Merck says chronic use or doses above the label should be carefully considered. The safe ivermectin dose for heartworm prevention is the one built into the correctly sized box.

The Zoetis-funded review says moxidectin has a greater margin of safety than ivermectin in MDR1 dogs, while the AHS, Merck, and Ohio State treat every labeled preventive dose as safe.

Heartworm medicine, side effects, and seizures

Can heartworm medication cause seizures in dogs? The clearest label warnings sit on the combinations: Simparica Trio and NexGard Plus, the two isoxazoline products, carry seizure cautions, and Trifexis has a caution for dogs with pre-existing epilepsy. Boehringer Ingelheim reports digestive and neurologic side effects and itching after Heartgard Plus.

For a dog with a seizure history, a heartworm-only chew avoids the isoxazoline caution entirely; make that choice with your vet, who knows the dog's history. The same isoxazoline question shapes the safest flea treatment for dogs.

Resistance is not a reason to skip prevention. Merck says resistant heartworms remain rare and geographically limited, for example to the Mississippi Delta, and Ohio State says resistance appears uncommon and is caught at annual screening.

What is the safest heartworm prevention medicine for dogs?

No single product is safest for every dog. The safest heartworm prevention is the macrocyclic lactone your vet prescribes after any needed heartworm test, at the labeled dose for your dog's weight. For most dogs, including herding breeds, that means a heartworm-only chew such as Heartgard Plus or Interceptor Plus, which also avoids the isoxazoline caution.

What dogs should not take Heartgard?

Heartgard Plus is labeled for dogs 6 weeks or older, and its maker says all dogs should be tested for heartworm before starting a preventive. That leaves three groups to hold off or check with the vet:

• Puppies younger than 6 weeks.
• Dogs 7 months or older that have not had a heartworm test, because Merck warns of hypersensitivity reactions in dogs with microfilaria. A heartworm-positive dog takes a preventive only within a vet-run treatment plan.
• MDR1 dogs given extra doses or combined with other P-glycoprotein-inhibiting drugs, where the AHS reports toxicities.

Does ivermectin cure heartworms in dogs?

No. The AHS says monthly preventive doses of a macrocyclic lactone take more than 2 years of continuous use before adult heartworms are 95% eliminated, while the disease keeps progressing, so it lists that approach alone as not recommended.

Merck says melarsomine is the only drug that kills adult heartworms, though chronic macrocyclic lactone use with doxycycline has also been effective. Merck adds that treated dogs are retested after 8 to 12 months, and the AHS says a year-round prevention program continues afterward.

Can you buy heartworm medicine without a vet prescription?

No. The AHS says heartworm preventives are available only by prescription from veterinarians, so there is no over-the-counter (OTC) heartworm medicine for dogs in the US and no best non-prescription option to choose. That applies to flea, tick, and heartworm combinations too.

The vet visit is required for a reason. The AHS says prescribing needs a licensed veterinarian with a valid client-patient relationship, and a dog without testing records has to be tested before any preventive is dispensed. Online and retail pharmacies fill heartworm meds against that prescription. The makers say the same thing:

• Simparica Trio: Zoetis says your veterinarian prescribes the correct dose.
• Revolution: Zoetis says it is available from select retailers and vets with a valid prescription.
• NexGard Plus: Boehringer Ingelheim says your dog needs a negative heartworm test and a prescription.
• ProHeart: the injection is given by your vet, and Zoetis requires vets and staff to be certified first.

If cost is the reason to look for heartworm medication without a vet, a few steps keep the prescription route affordable and safe:

• Ask your vet for a written prescription you can compare across pharmacies.
• Buy only from a seller that asks for that prescription, since the AHS says no heartworm preventive is sold without one.
• Order the weight band on the prescription, and check that the label lists heartworm disease.

Can I buy ivermectin for my dog without a vet?

Not for heartworm prevention. Ivermectin preventives such as Heartgard Plus are prescription-only, and the AHS requires testing before dispensing when records are missing. A farm or livestock ivermectin product is not a safe stand-in, given the AHS link to poisonings in sensitive dogs, and we found no FDA guidance on a livestock-ivermectin dose for dogs.

Does Tractor Supply sell heartworm pills for dogs?

Tractor Supply's current heartworm stock could not be verified from its own pages, so ask your local store. At every farm store, pet store, and online seller, the AHS rule applies: heartworm preventives are available only by prescription from veterinarians. A heartworm pill sold without one is not a heartworm preventive.

Heartworm pills vs topicals vs the ProHeart injection

Heartworm preventives come in three forms, the AHS says: monthly chewable pills, monthly topical spot-on liquids, and an injection given every 6 or 12 months. Merck's advice is to choose the injectable, topical, or oral preventive that fits your routine, then give it year-round.

How the three forms compare:

• Pills and chews: monthly, the widest product choice, and the dog has to eat the dose.
• Topicals: monthly, nothing to swallow, and limited to moxidectin and selamectin products.
• Injection: every 6 or 12 months at the clinic, with no monthly doses to forget.

Monthly pills, chews, and tablets

The AHS guidelines list ivermectin and milbemycin oxime for monthly oral use, and newer chews such as Simparica Trio and NexGard Plus deliver moxidectin by mouth. Some formulations are flavored chews made to improve acceptance; Trifexis is a beef-flavored tablet.

Dogs need these heartworm tablets every month, on the same date, and a flea, tick, and heartworm pill such as Simparica Trio covers all three in one chew. Pills suit dogs that take treats readily and owners who prefer a monthly routine at home.

Topical heartworm prevention

Moxidectin and selamectin are the topical liquid options, on the same monthly schedule as oral products, according to the AHS. Does Revolution for dogs cover heartworm? Yes: Zoetis says one monthly dose applied at the base of the neck in front of the shoulder blades protects against heartworm disease, fleas, and other parasites.

Topicals are the main option for pill refusers that will not take a flavored chew, and they suit owners who would rather apply a dose than hide one in food.

The ProHeart 6 and ProHeart 12 heartworm injection

The AHS says one slow-release injection of moxidectin microspheres gives 6 months of continuous protection and can improve compliance. The FDA says ProHeart 12 uses the same formulation with three times the moxidectin, for a 12-month duration, and studies showed it 100 percent effective in preventing heartworm disease for one year.

Zoetis lists ProHeart 6 for dogs 6 months or older and ProHeart 12 for dogs 12 months or older, given by a certified vet. Zoetis says not to give it to dogs that are:

• Sick or debilitated.
• Underweight, or have a history of weight loss.
• Previously found to be hypersensitive to the drug.

The shot's history is part of the decision. The FDA says ProHeart 6 was approved in 2001, then voluntarily removed in 2004 after reports of adverse events including death, anaphylaxis, seizures, elevated liver enzymes, and immune-related blood problems. It later returned under a risk-minimization plan.

ProHeart 12 safety work included a study in ivermectin-sensitive collies, the FDA notes. For scheduling and cost details, see heartworm injection for dogs.

What to do after a missed dose

Each maker gives its own instructions, so read the label for your product:

• Simparica Trio: Zoetis says to give a new dose immediately, then continue the monthly schedule.
• Revolution: Zoetis says to give the missed dose as soon as you remember, then resume monthly doses, and never give a double dose.
• Sentinel Spectrum: the label cautions that fewer than 6 monthly doses after the last mosquito exposure may not give complete protection.

The ProHeart injection sidesteps the problem, since one vet-given shot covers 6 or 12 months.

Best heartworm medicine for dogs questions