Generic Name: Pyrimethamine Brand Name(s): Generics only. No brands available.


What is Pyrimethamine used for?

Pyrimethamine is used with other medication (such as a sulfonamide) to treat a serious parasite infection (toxoplasmosis) of the body, brain, or eye or to prevent toxoplasmosis infection in people with HIV infection. Pyrimethamine belongs to a class of drugs known as antiparasitics. It works by killing parasites.

How should I take Pyrimethamine?

Take this medication by mouth usually once or twice daily or as directed by your doctor. Take this medication with food to decrease nausea and vomiting. If vomiting is severe or continues, your doctor may lower your dose or direct you to stop taking this medication. Your doctor will prescribe another medication (folic/folinic acid) to prevent blood problems caused by pyrimethamine. Follow your doctor’s directions carefully. Drink plenty of fluids to prevent kidney problems if you are taking a “sulfa” medication with pyrimethamine.

This medication works best when the amount of drug in your body is kept at a constant level. Therefore, take this drug and other antiparasitic drugs regularly, exactly as prescribed by your doctor. To help you remember, take it at the same time(s) each day.

Dosage is based on the type of infection, your medical condition, age, and response to treatment. The length of time you will take this medication depends on your infection. Your dose must be carefully adjusted by your doctor to treat your infection and prevent serious side effects. Follow your doctor’s directions carefully.

Do not take more or less of this drug than prescribed. Do not stop taking it before completing this prescription unless directed to do so by your doctor, even if you feel better. Skipping or changing your dose without approval from your doctor may cause the amount of parasites to increase, make the infection more difficult to treat (resistant), or worsen side effects.

Tell your doctor if your condition persists or worsens.

How do I store Pyrimethamine?

Pyrimethamine is best stored at room temperature away from direct light and moisture. To prevent drug damage, you should not store Pyrimethamine in the bathroom or the freezer. There may be different brands of Pyrimethamine that may have different storage needs. It is important to always check the product package for instructions on storage, or ask your pharmacist. For safety, you should keep all medicines away from children and pets.

You should not flush Pyrimethamine down the toilet or pour them into a drain unless instructed to do so. It is important to properly discard this product when it is expired or no longer needed. Consult your pharmacist for more details about how to safely discard your product.

Precautions & warnings

What should I know before using Pyrimethamine?

Before taking pyrimethamine, tell your doctor or pharmacist if you are allergic to it; or if you have any other allergies. This product may contain inactive ingredients, which can cause allergic reactions or other problems. Talk to your pharmacist for more details.

Before using this medication, tell your doctor or pharmacist your medical history, especially of: seizures, kidney problems, liver problems, a certain type of low red blood cell count (megaloblastic anemia due to low blood folate), low folic acid levels from other conditions (such as malnutrition, problems with absorption of food, alcoholism), low red/white blood cell counts, low blood-clotting cell (platelet) count.

Before having surgery, tell your doctor or dentist about all the products you use (including prescription drugs, nonprescription drugs, and herbal products).

During pregnancy, this medication should be used only when clearly needed. Discuss the risks and benefits with your doctor. Folic acid is very important during pregnancy. Your doctor will prescribe folic/folinic acid to prevent low folate levels.

This medication passes into breast milk. Consult your doctor before breast-feeding.

Is it safe during pregnancy or breast-feeding?

There are no adequate studies in women for determining risk when using this Pyrimethamine during pregnancy or while breastfeeding. Please always consult with your doctor to weigh the potential benefits and risks before taking Pyrimethamine. Pyrimethamine is pregnancy risk category C according to the US Food and Drug Administration (FDA).

FDA pregnancy risk category reference below:

  • A=No risk,
  • B=No risk in some studies,
  • C=There may be some risk,
  • D=Positive evidence of risk,
  • X=Contraindicated,
  • N=Unknown

Side effects

What side effects can occur from Pyrimethamine?

Nausea, vomiting, and loss of appetite may occur. If any of these effects persist or worsen, tell your doctor or pharmacist promptly.

Remember that your doctor has prescribed this medication because he or she has judged that the benefit to you is greater than the risk of side effects. Many people using this medication do not have serious side effects.

Some people using this medication may develop serious side effects including blood problems, especially at higher doses. This risk can be reduced with the use of folic/folinic acid and regular blood tests. Tell your doctor right away if any of these symptoms occur: easy bruising/bleeding, signs of serious infection (such as high fever, severe chills, persistent sore throat), signs of low red blood cell count (such as severe tiredness, pale lips/nails/skin, fast heartbeat/breathing with usual activities), swollen/painful tongue.

Get medical help right away if you have any very serious side effects, including: bloody/pink urine, chest pain, slow/fast/irregular heartbeat.

A very serious allergic reaction to this drug is rare. However, get medical help right away if you notice any symptoms of a serious allergic reaction, including: rash, itching/swelling (especially of the face/tongue/throat), severe dizziness, trouble breathing.

Not everyone experiences these side effects. There may be some side effects not listed above. If you have any concerns about a side-effect, please consult your doctor or pharmacist.


What drugs may interact with Pyrimethamine?

Some products that may interact with this drug include: lorazepam, penicillamine, sulfa drugs (such as sulfamethoxazole), drugs that can lower folate levels (such as phenytoin, trimethoprim), drugs that can lower blood counts (such as proguanil, zidovudine, chemotherapy including methotrexate, daunorubicin, cytosine).

Pyrimethamine may interact with other drugs that you are currently taking, which can change how your drug works or increase your risk for serious side effects. To avoid any potential drug interactions, you should keep a list of all the drugs you are using (including prescription drugs, nonprescription drugs and herbal products) and share it with your doctor and pharmacist. For your safety, do not start, stop, or change the dosage of any drugs without your doctor’s approval.

Does food or alcohol interact with Pyrimethamine?

Pyrimethamine may interact with food or alcohol by altering the way the drug works or increase the risk for serious side effects. Please discuss with your doctor or pharmacist any potential food or alcohol interactions before using this drug.

What health conditions may interact with Pyrimethamine?

Pyrimethamine may interact with your health condition. This interaction may worsen your health condition or alter the way the drug works. It is important to always let your doctor and pharmacist know all the health conditions you currently have.


The information provided is not a substitute for any medical advice. You should ALWAYS consult with your doctor or pharmacist before using this Pyrimethamine.

What is the dose of Pyrimethamine for an adult?

Usual Adult Dose for Malaria Prophylaxis

25 mg orally once a week. Prophylaxis should begin one week prior to departure and continue for at least 6 to 10 weeks following exposure.

Usual Adult Dose for Toxoplasmosis

Initially: 50 to 75 mg orally once a day with 1 to 4 g of a sulfonamide (e.g., sulfadoxine, sulfadiazine). Continue for 1 to 3 weeks, depending on response and tolerance. Dosage for each drug may then be reduced by one-half and continued for an additional 4 or 5 weeks. Patients receiving pyrimethamine should also receive folinic acid.

Usual Adult Dose for Toxoplasmosis – Prophylaxis

1 mg/kg or 15 mg/m2 (max 25 mg) orally every day plus folinic acid (leucovorin) 5 mg orally every 3 days plus sulfadiazine 85 to 120 mg/kg/day divided into 2 to 4 oral doses. Clindamycin 20 to 30 mg/kg/day may be used in place of sulfadiazine as an alternative regimen.

Usual Adult Dose for Pneumocystis Pneumonia Prophylaxis

50 to 75 mg orally once a week. Pyrimethamine is used in combination with dapsone and leucovorin. This is considered an alternative regimen for patients who do not tolerate trimethoprim-sulfamethoxazole.


Pyrimethamine is a folate antagonist and is contraindicated in patients with megaloblastic anemia caused by folate deficiency.

Patients being treated for toxoplasmosis with high doses of pyrimethamine should be monitored for signs or symptoms and laboratory abnormalities suggestive of folate deficiency. If folate deficiency occurs, the dosage should be reduced or the drug discontinued, depending on the response of the patient. Folinic acid (leucovorin) should be administered until normal hematopoiesis is restored.

Pyrimethamine may be carcinogenic. Pyrimethamine therapy has been reported to produce a significant increase in the number of lung tumors in mice when given intraperitoneally at doses of 25 mg/kg.

The recommended dosage of chemoprophylaxis of malaria should not be exceeded. A small “starting” dose for toxoplasmosis is advised in patients with convulsive disorders to avoid the potential nervous system toxicity of pyrimethamine. Pyrimethamine should be administered with caution in patients with impaired renal or hepatic function or in patients with possible folate deficiency, such as individuals with malabsorption syndrome, alcoholism, or pregnancy.

Semiweekly blood and platelet counts are recommended for patients being treated for toxoplasmosis.

Patients should be warned to discontinue pyrimethamine therapy if a rash develops and to promptly seek medical attention. Additionally, patients should be advised to contact their physician if sore throat, pallor, purpura, and/or glossitis develop.

The drug may be taken with meals to minimize anorexia and vomiting.

Pyrimethamine therapy should be kept out of the reach of infants and children as they are extremely susceptible to adverse effects from an overdose. Deaths in pediatric patients have been associated after accidental ingestion.

Clinical studies of pyrimethamine therapy did not include sufficient numbers of patients aged 65 and over to determine whether they respond differently from younger patients. Other observed clinical experience has not identified differences in responses between the elderly and younger patients. In general, dose selection for elderly patients should be cautious, usually starting at low end of the dosing range, reflecting the greater frequency of decreased hepatic, renal, or cardiac function, and of concomitant disease or other drug therapy.

Other Comments

Pyrimethamine may be taken with meals.

What is the dose of Pyrimethamine for a child?

Usual Pediatric Dose for Malaria Prophylaxis

Less than 4 years: 6.25 mg orally once a week.

4 to 10 years: 12.5 mg orally once a week.

Usual Pediatric Dose for Toxoplasmosis

Newborns and infants:

Initial: 2 mg/kg/day orally divided every 12 hours for 2 days, then 1 mg/kg/day once daily given with sulfadiazine for the first 6 months; next 6 months: 1 mg/kg/day 3 times per week with sulfadiazine; oral folinic acid 5 to 10 mg 3 times per week should be administered to prevent hematological toxicity.

1 to 12 years: 2 mg/kg/day divided every 12 hours for 3 days followed by 1 mg/kg/day (maximum 25 mg/day) once daily or divided twice daily for 4 weeks given with sulfadiazine; oral folinic acid 5 to 10 mg 3 times per week should be administered to prevent hematological toxicity.

How is Pyrimethamine available?

Pyrimethamine is available in the following dosage forms and strengths:

  • Oral tablet,
  • Compounding powder.

What should I do in case of an emergency or overdose?

In case of an emergency or an overdose, call your local emergency services or go to your nearest emergency room.

What should I do if I miss a dose?

If you miss a dose of Pyrimethamine, take it as soon as possible. However, if it is almost time for your next dose, skip the missed dose and take your regular dose as scheduled. Do not take a double dose.

Hello Health Group does not provide medical advice, diagnosis or treatment.

Review Date: April 21, 2018 | Last Modified: April 21, 2018

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