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Pyrimethamine is used to treat a parasitic infection called toxoplasmosis. The medicine kills parasites by stopping the production of substances they need to grow and reproduce. Common side effects include loss of appetite and vomiting. Keep all appointments to have your blood checked. Call your health care provider right away if you have a rash, fever, sore throat, pale skin, or tongue swelling. 

Uses

What is pyrimethamine used for?

Pyrimethamine is commonly used to treat toxoplasmosis, which is an infection caused by a parasite called Toxoplasma gondii. 

Pyrimethamine may also be used for other conditions as determined by your health care provider.

How does pyrimethamine work (mechanism of action)?

Pyrimethamine kills certain types of parasites by blocking their ability to create proteins and other substances that they need to grow and reproduce.

How is pyrimethamine supplied (dosage forms)?

Brands and other names

  • Daraprim

Dosage forms and strengths

  • 25 mg oral tablets

How should I store pyrimethamine?

Pyrimethamine should be stored at room temperature, between 59 F and 77 F (15 C and 25 C). Store in a cool, dry place. Protect the medicine from light.

Side Effects

What are the most common side effects of pyrimethamine?

The most common side effects of pyrimethamine are listed below. Tell your health care provider if you have any of these side effects that bother you.

  • Loss of appetite
  • Vomiting
  • Blood disorders (see below)
  • Bald tongue
  • Blood in your urine (pee)
  • Heart rhythm changes (see below)

There may be other side effects of pyrimethamine that are not listed here. Contact your health care provider if you think you are having a side effect of a medicine. In the U.S., you can report side effects to the FDA at www.fda.gov/medwatch or by calling 800-FDA-1088 (800-332-1088). In Canada, you can report side effects to Health Canada at www.health.gc.ca/medeffect or by calling 866-234-2345.

What are the serious side effects of pyrimethamine?

While less common, the most serious side effects of pyrimethamine are described below, along with what to do if they happen.

Severe Allergic Reactions. Pyrimethamine may cause allergic reactions, which can be serious. Stop taking pyrimethamine and get help right away if you have any of the following symptoms of a serious allergic reaction.

  • Breathing problems or wheezing
  • Racing heart
  • Fever or general ill feeling
  • Swollen lymph nodes
  • Swelling of the face, lips, mouth, tongue, or throat
  • Trouble swallowing or throat tightness
  • Itching, skin rash, or pale red bumps on the skin called hives
  • Nausea or vomiting
  • Dizziness, feeling lightheaded, or fainting
  • Stomach cramps
  • Joint pain

Severe Skin Reactions. Pyrimethamine can cause severe skin reactions, including Stevens-Johnson syndrome (SJS), toxic epidermal necrolysis (TEN), and erythema multiforme (EM), that can lead to death if not treated. If you get a rash, stop pyrimethamine and call your health care provider right away. Get emergency help if you have any of the following symptoms of SJS, TEN, or EM.

  • Painful red or purple skin that looks burned and peels off
  • Flat red rash or blisters on your skin, mouth, nose, and genitals
  • Red, painful, watery eyes or dry, bloodshot eyes
  • Flu-like symptoms including fever, chills, and muscle aches
  • Red spots, wheals, or blisters on your hands, arms, face, neck, legs, or trunk
  • Joint pain
  • Vision changes

Blood Disorders. Pyrimethamine can cause blood disorders, such as megaloblastic anemia, low white blood cell levels (leukopenia), low platelet levels (thrombocytopenia), and low neutrophil levels (neutropenia). Stop using pyrimethamine and get help right away if you have any of the following symptoms of blood disorders.

  • Fever
  • Shortness of breath
  • Pale or yellowish skin
  • Easy bruising or bleeding
  • Frequent infection
  • Unusual weakness or tiredness
  • Dizziness, lightheadedness, or feeling like you are about to pass out
  • Headache
  • Fast or abnormal heartbeat

Heart Rhythm Changes. Pyrimethamine may cause heart rhythm problems that can lead to an irregular heartbeat and fainting. In rare cases, this may cause the heart to suddenly stop beating (cardiac arrest). Some people have a higher risk of this, including people with existing heart disease or who take other medicines that affect the heart. Get emergency help if you have any of the following signs of heart rhythm changes.

  • Chest pain
  • Shortness of breath
  • Feeling dizzy, lightheaded, or fainting
  • Changes in your heart rate or rhythm, such as a fast, slow, pounding, or skipping heartbeat

Lung Problems. Pyrimethamine can cause pulmonary eosinophilia. Pulmonary eosinophilia is a lung problem caused by a high number of a type of white blood cell (eosinophils). Too many eosinophils can cause swelling and damage to the lungs, which can lead to breathing problems and even death. Stop using pyrimethamine and get help right away if you have any of the following symptoms of pulmonary eosinophilia.

  • Fever or night sweats
  • Shortness of breath
  • Cough
  • Chest pain when breathing
  • Muscle pain
  • Weight loss

Warnings & Precautions

Who should not use pyrimethamine?

Allergies to Ingredients. People who are allergic to any of the following should not take pyrimethamine.

  • Pyrimethamine
  • Daraprim
  • Any of the ingredients in the specific product dispensed

Your pharmacist can tell you all of the ingredients in the specific pyrimethamine products they stock.

Severe Folate (Folic Acid) Deficiency. Do not take pyrimethamine if you have a history of megaloblastic anemia due to folate deficiency. This is a condition where you have a low red blood cell count due to low levels of vitamin B9 (folate). Your health care provider may do blood tests before you start taking pyrimethamine to make sure you do not have this condition.

What should I know about pyrimethamine before using it?

Do not take pyrimethamine unless it has been prescribed to you by a health care provider. Take it as prescribed. If your health care provider prescribes other medicines, such as leucovorin or other antibiotics, to take with pyrimethamine, make sure to also take it as prescribed.

Do not share pyrimethamine with other people, even if they have the same condition as you. It may harm them.

Keep pyrimethamine out of the reach of children. 

Pyrimethamine can be taken with meals to reduce stomach upset, loss of appetite, or vomiting that may happen after taking the medicine.

What should I tell my health care provider before using pyrimethamine?

Tell your health care provider about all of your health conditions and any prescription or over-the-counter (OTC) medicines, vitamins/minerals, herbal products, and other supplements you are using. This will help them find out if pyrimethamine is right for you.

In particular, make sure that you discuss any of the following.

Current and Past Health Conditions. Tell your health care provider if you have any of the following.

Other Medicines and Supplements. Pyrimethamine may interact with other medicines and supplements. Before taking pyrimethamine, tell your health care provider about any prescription or over-the-counter (OTC) medicines, vitamins/minerals, herbal products, and other supplements you are using. See the Interactions section for more details.

Pregnancy. It is not known if or how pyrimethamine could affect pregnancy or harm an unborn baby. Tell your health care provider if you are pregnant or plan to become pregnant. Your health care provider will let you know if you should take pyrimethamine while you are pregnant or trying to get pregnant.

Breastfeeding. Pyrimethamine passes into breast milk. Tell your health care provider if you are breastfeeding or plan to breastfeed. Your health care provider will advise you if you should stop breastfeeding or stop pyrimethamine.

Interactions

Does pyrimethamine interact with foods or drinks?

There are no known interactions between pyrimethamine and foods or drinks.

There are no known interactions between pyrimethamine and alcohol, but heavy drinking can slow your immune response and make it harder for your body to fight infection.

Does pyrimethamine interact with other medicines (drug interactions)?

Always tell your health care provider about any prescription or over-the-counter (OTC) medicines, vitamins/minerals, herbal products, and other supplements you are using.

In particular, make sure that you discuss if you are using any of the following before taking pyrimethamine.

  • A sulfonamide, such as sulfamethoxazole/trimethoprim (Bactrim, Septra, Sulfatrim), which is an antibiotic used to treat certain infections
  • Proguanil, which is a medicine used for malaria
  • Zidovudine, which is a medicine used for HIV
  • Methotrexate (Trexall and others), which is a medicine commonly used for rheumatoid arthritis or certain types of cancer
  • Lorazepam (Ativan), which is a medicine used for anxiety and seizures
  • Phenytoin (Dilantin, Phenytek, and others), which is a medicine to control seizures

This may not be a complete list of medicines that can interact with pyrimethamine. Always check with your health care provider.

Does interact with other drugs you are taking?

Enter your medication into the WebMD interaction checker

Overdose/Missed Dose

What should I do if I accidentally use too much pyrimethamine?

If you or someone else has used too much pyrimethamine, get medical help right away, call 911, or contact a Poison Control center at 800-222-1222.

What should I do if I miss a dose of pyrimethamine?

If you miss a dose, take it as soon as you remember. If it is almost time for your next dose, skip the missed dose and only take the next dose. Do not take double or extra doses.

It is important to complete your prescribed course of pyrimethamine, even if your symptoms get better or go away. This will reduce the risk that future infections are resistant to pyrimethamine or other similar medicines.