Milbemycin Oxime

By Lauren R. Forsythe, PharmD, MBA, DICVP; Rania Gollakner, BS, DVM, MPH

What is milbemycin oxime?

Milbemycin oxime (brand name Interceptor®) is a heartworm disease preventive and treats internal parasites (e.g., hookworms, roundworms) in dogs and cats.

It is also found in combination products (such as Sentinel®, Sentinel Spectrum®, and Trifexis®) with other drugs (lufenuron or spinosad). In addition to treating internal parasites and preventing heartworm, these combination products are flea preventives as well.

Milbemycin is also used “off label” or “extra-label” to treat mite infestations in dogs. Many drugs are commonly prescribed for off-label use in veterinary medicine. In these instances, follow your veterinarian’s directions and cautions carefully.

How is milbemycin oxime given?

  • Milbemycin is given by mouth with a flavored oral tablet. 
  • It can be given with or without food. 
  • If your pet acts sick or vomits after getting milbemycin on an empty stomach, try giving with a small amount of food.

This medication should take effect within 1 to 2 days for internal parasites, and within 24 hours for external parasites, and improvements in clinical signs should follow.

What if I miss giving my pet the medication (or my shipment is late)?

Give the missed dose as soon as you remember and then wait the amount of time between doses recommended by your veterinarian before giving the next dose. Do not give your dog two doses at once, or give extra doses.

If you are using milbemycin as a heartworm preventive and more than 8 weeks have passed without giving this medication, contact your veterinarian for guidance.

Are there any potential side effects?

At regular doses, milbemycin is well-tolerated. When used at higher doses for treating mites, side effects have been observed. If any of the following are observed, contact your veterinarian immediately:

  • depression, lack of energy, or weakness
  • stumbling or collapse
  • seizures or coma
  • dilated (big) pupils
  • excessive drooling

Some breeds, such as collies, sheepdogs, and collie- or sheepdog-cross breeds, are sensitive to only moderate doses of milbemycin and may have side effects at lower doses.

This moderate-acting medication should stop working in a few days, although effects can be longer in pets with liver or kidney disease.

Are there any risk factors for this medication?

Milbemycin is safe for most pets. Milbemycin should not be used in:

  • dogs without a current negative heartworm test
  • puppies less than four weeks of age or puppies or dogs weighing less than 2 lb (900 g)
  • cats less than six weeks of age or in cats or kittens weighing less than 1.5 lb (680 g)

Your veterinarian will advise you on the safety of milbemycin use in your dog.

Are there any drug interactions I should be aware of?

Other drugs can interact with milbemycin (e.g., cyclosporine, diltiazem, azole antifungals, erythromycin). Be sure to tell your veterinarian about any medications (including vitamins, supplements, or herbal therapies) that your pet is taking.

Is any monitoring needed with this medication?

Your pet should be tested for heartworm before receiving any product containing milbemycin. If your pet has a high load of immature worms and is given milbemycin, a life-threatening reaction can occur.

For breeds of dogs with potential milbemycin sensitivity, screening may be done ahead of receiving the drug. DNA testing can determine whether a pet has the genetic mutation responsible for adverse reactions to milbemycin.

How do I store milbemycin?

  • Store milbemycin oxime-based products at room temperature.
  • Store in a cool, dry place, away from heat and direct sunlight. 
  • Do not store this medicine in the bathroom, near the kitchen sink, or in a damp place. Exposure to heat or moisture may reduce the drug’s effectiveness.

What should I do in case of emergency?

If you suspect an overdose or an adverse reaction to the medication, call your veterinary office immediately. If they are not available, follow their directions in contacting an emergency facility.

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