This medication 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.

This section contains uses of this drug that are not listed in the approved professional labeling for the drug but that may be prescribed by your health care professional. Use this drug for a condition that is listed in this section only if it has been so prescribed by your health care professional.

This drug may also be used with other medications (such as dapsone) for pneumocystis pneumonia in people with AIDS.

Take this medication by mouth as directed by your doctor, usually once or twice daily. 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. 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 lasts or gets worse.

See also How to Use section.

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

Remember that this medication has been prescribed because your doctor 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 you have any serious side effects, including:

• easy bruising/bleeding
• signs of serious infection (such as sore throat that doesn't go away, high fever, severe chills)
• 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

This is not a complete list of possible side effects. If you notice other effects not listed above, contact your doctor or pharmacist.

In the US - Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088 or at www.fda.gov/medwatch.

In Canada - Call your doctor for medical advice about side effects. You may report side effects to Health Canada at 1-866-234-2345.

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 breastfeeding.

Drug interactions may change how your medications work or increase your risk for serious side effects. This document does not contain all possible drug interactions. Keep a list of all the products you use (including prescription/nonprescription drugs and herbal products) and share it with your doctor and pharmacist. Do not start, stop, or change the dosage of any medicines without your doctor's approval.

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)

This medication can slow down the removal of other medications from your body, which may affect how they work. An example of an affected drug is dofetilide, among others.

If someone has overdosed and has serious symptoms such as passing out or trouble breathing, call 911. Otherwise, call a poison control center right away. US residents can call 1-800-222-1222. Canada residents can call 1-844-764-7669. Symptoms of overdose may include: abdominal pain, severe/repeated vomiting, vomiting blood, seizures, slow/shallow breathing, inability to wake up.

Do not share this medication with others.

Lab and/or medical tests (such as complete blood count, liver function) should be done while you are taking this medication. Keep all medical and lab appointments. Consult your doctor for more details.

If you miss a dose, take it as soon as you remember. If it is near the time of the next dose, skip the missed dose. Take your next dose at the regular time. Do not double the dose to catch up.

Store at room temperature away from light and moisture. Do not store in the bathroom. Keep all medications away from children and pets.

Do not flush medications down the toilet or pour them into a drain unless instructed to do so. Properly discard this product when it is expired or no longer needed. Consult your pharmacist or local waste disposal company.