By Rania Gollakner, BS, DVM, MPH

What is pyrimethamine?

Pyrimethamine (brand name: Daraprim®) is a systemic antiprotozoal medication used to treat parasitic infections caused by Hepatozoon americanum (a tick-borne disease) and Toxoplasma gondii (a parasite that causes the infection toxoplasmosis).

Its use in cats and dogs to treat certain parasitic infections is 'off label' or 'extra label'. Many drugs are commonly prescribed for off label use in veterinary medicine. In these instances, follow your veterinarian’s directions and cautions very carefully as their directions may be significantly different from those on the label.

How do I give my pet pyrimethamine?

Pyrimethamine is given by mouth as a tablet or it can be given as a compounded liquid medication. It can be given with or without food. This medication has an unpleasant taste, so mixing the medication into food is not recommended. Pregnant women or those that may become pregnant should handle this medication carefully.

It will take a few days before noting any effects, and the medications full effects may not be obvious for several weeks.

What if I miss giving my pet the medication?

If you miss a dose, give it when you remember, but if it is close to the time for the next dose, skip the dose you missed and give it at the next scheduled time, and return to the regular dosing schedule. Never give your pet two doses at once or give extra doses.

Are there any potential side effects?

Side effects may include anorexia, tiredness/lack of energy, vomiting, depression. In cats, side effects may be more severe. This medication can also decrease blood cell counts and cause inaccurate results when testing for vitamin B12 and folic acid.

This medication is a short-acting drug, and therefore will stop working within 24 hours. In pets with kidney or liver disease, the medication’s actions can last a bit longer.

Are there any risk factors for this medication?

Pyrimethamine should not be used in pets that are allergic to it. It should be used with caution in pets with a folate deficiency, a preexisting blood disorder, or liver or kidney disease. It should also be used with caution in cats because side effects in cats can be severe.

Pyrimethamine should be used with caution in pregnant or breeding animals, and the risk versus the benefit must be weighed. It should not be used in lactating animals and a milk replacer should be utilized if use of the medication is necessary.

Are there any drug interactions I should be aware of?

Certain medications should be used cautiously with pyrimethamine including dapsone, folic acid, para-aminobenzoic acid, sulfonamides, trimethoprim, and zidovudine.

Be sure to tell your veterinarian about any medications (including vitamins, supplements, or herbal therapies) that your pet is taking.

Is there any monitoring that needs to be done with this medication?

Your veterinarian will monitor your pet to be sure the medication is working and will take blood samples to monitor for low blood cell counts. Your veterinarian may monitor your pet to be sure that the medication is working.

Monitor your pet for bleeding, extreme lethargy, or fever, and contact your veterinarian immediately if these signs are noted.

How do I store pyrimethamine?

The pill form of this medication should be stored at 15°- 25°C (59°-77°F), away from light and in a tight container. If compounded into a liquid suspension, it can be stored at room temperature for 7 days, and then should be discarded.

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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