Pyrimethamine is an antiparasite medicine that helps prevent parasites from growing and reproducing in the body.

Pyrimethamine is used in adults and children to treat or prevent certain types of malaria. However, pyrimethamine is generally not preferred as a medicine to prevent malaria while traveling. When used to treat malaria, pyrimethamine should be used together with a faster-acting anti-malaria medicine such as chloroquine or quinine.

Pyrimethamine is also used to treat toxoplasmosis, an infection caused by the Toxoplasma parasite.

Pyrimethamine may also be used for purposes not listed in this medication guide.

You should not use pyrimethamine if you have a blood cell disorder called megaloblastic anemia that has been caused by folate deficiency.

Follow all directions on your prescription label. Your doctor may occasionally change your dose. Do not take this medicine in larger or smaller amounts or for longer than recommended.

Your dosage and the length of time you take pyrimethamine will depend on the reason you are taking this medicine. In some cases pyrimethamine is taken for several weeks, and you may need to take the medicine only once per week.

The pyrimethamine dose for treating toxoplasmosis is much higher than the dose for malaria.

Follow your doctor's dosing instructions very carefully.

Your dose may need to be cut in half after you have been taking pyrimethamine for 1 to 3 weeks (after 2 to 4 days for a child).

Take with food if pyrimethamine upsets your stomach or affects your appetite.

Pyrimethamine is often given in combination with other medications. Use all medications as directed by your doctor. Read the medication guide or patient instructions provided with each medication. Do not change your doses or medication schedule without your doctor's advice.

While using pyrimethamine, you may need frequent blood tests.

Store at room temperature away from moisture, heat, and light.

You should not use pyrimethamine if you are allergic to it, or if you have:

• a blood cell disorder called megaloblastic anemia that has been caused by folate (folic acid) deficiency.

To make sure pyrimethamine is safe for you, tell your doctor if you have ever had:

• seizures (especially if you take phenytoin);

• liver disease;

• kidney disease;

• alcoholism; or

• if you are malnourished.

Using pyrimethamine during pregnancy could harm the unborn baby. Use effective birth control to prevent pregnancy while taking pyrimethamine, and tell your doctor if you become pregnant.

In some cases, pyrimethamine is given to pregnant women to treat toxoplasmosis. Talk with your doctor about the risks and benefits of using this medicine if you are pregnant.

Pyrimethamine can pass into breast milk and may cause side effects in the nursing baby. Tell your doctor if you are breast-feeding.

Take the missed dose as soon as you remember. Skip the missed dose if it is almost time for your next scheduled dose. Do not take extra medicine to make up the missed dose.

Seek emergency medical attention or call the Poison Help line at 1-800-222-1222. An overdose of pyrimethamine can be fatal, especially to a child.

Overdose symptoms may include stomach pain, severe vomiting, coughing up blood or vomit that looks like coffee grounds, feeling anxious or excited, seizure (convulsions), and weak or shallow breathing (breathing may stop).

Follow your doctor's instructions about any restrictions on food, beverages, or activity.

Get emergency medical help if you have signs of an allergic reaction: hives; difficult breathing; swelling of your face, lips, tongue, or throat.

Pyrimethamine may cause serious side effects. Stop using pyrimethamine and call your doctor at once if you have:

• sore throat, swelling in your tongue;

• pale skin, easy bruising, purple spots under your skin;

• the first sign of any skin rash, no matter how mild;

• blood in your urine;

• fever, cold or flu symptoms;

• new or worsening cough, fever, trouble breathing;

• irregular heartbeats;

• signs of folate deficiency--unusual tiredness, mood changes, feeling sick, mouth sores, stomach discomfort, loss of appetite;

• severe skin reaction--fever, sore throat, swelling in your face or tongue, burning in your eyes, skin pain, followed by a red or purple skin rash that spreads (especially in the face or upper body) and causes blistering and peeling.

Common side effects of pyrimethamine may include:

• vomiting; or

• loss of appetite.

This is not a complete list of side effects and others may occur. Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088.

Tell your doctor about all your current medicines and any you start or stop using, especially:

• methotrexate;

• phenytoin; or

• a sulfa drug.

This list is not complete. Other drugs may interact with pyrimethamine, including prescription and over-the-counter medicines, vitamins, and herbal products. Not all possible interactions are listed in this medication guide.