Indications
Pyrimethamine is an antiparasitic medicine. In people it is mainly used, together with a sulfonamide such as Sulfadiazine, to treat toxoplasmosis, an infection caused by the parasite Toxoplasma gondii that can be serious in pregnancy, in newborns and in people with weak immunity. In combination with Sulfadoxine it has also been used against malaria, although resistance now limits this use. Some combination products containing Pyrimethamine and a sulfonamide are made for animals (for example to treat coccidiosis in poultry); those products are for veterinary use only.
How It Works
Pyrimethamine blocks an enzyme called dihydrofolate reductase in the parasite. The parasite needs this enzyme to make folic acid, which it uses to copy its DNA and multiply. Sulfonamides block an earlier step in the same pathway, so the two medicines work much better together.
Dosage and Administration
Doses for toxoplasmosis are high, vary with the patient and must be set by a doctor; take exactly as prescribed, usually for several weeks. Folinic acid (Calcium Folinate) is normally given at the same time to protect the bone marrow. Children’s doses are based on body weight. Take with food to reduce stomach upset. Veterinary products should be used only as directed on the label or by a veterinarian, and are not for human use.
Interactions
Other medicines that lower folate or affect the bone marrow, such as Methotrexate, Trimethoprim, Co-trimoxazole, Proguanil and Zidovudine, increase the risk of blood problems. Lorazepam has been linked to liver problems when combined with it. Folic acid (as opposed to folinic acid) may reduce its effect against the parasite.
Contraindications
Do not take if you are allergic to Pyrimethamine, or if you have megaloblastic anaemia caused by folate deficiency. Combination products containing a sulfonamide must not be used by people allergic to sulfa medicines.
Side Effects
- Loss of appetite, nausea and vomiting
- Low blood counts (anaemia, low white cells or platelets), especially at high doses
- Skin rash
- Headache and dizziness
Get urgent medical help if you have a sore throat, fever, mouth ulcers, unusual bruising or bleeding, extreme tiredness or paleness, a skin rash with blistering or peeling, or fits.
Pregnancy and Lactation
Pyrimethamine may be used in pregnancy to treat toxoplasmosis only under specialist care, usually after the first 3 months and with folinic acid. It passes into breast milk; breastfeeding should be discussed with the doctor.
Precautions and Warnings
Regular blood counts are needed during treatment, often weekly or twice weekly. Use with care in people with epilepsy, liver or kidney disease, or a history of folate deficiency or asthma. Keep tablets away from children, as accidental swallowing can be very dangerous. Veterinary products must observe the withdrawal period on the label before meat or eggs are used for food.
Overdose
Overdose can cause vomiting, fits, breathing problems and severe blood disorders, and can be fatal in children. Seek emergency medical help immediately.
Therapeutic Class
Antiprotozoal and antimalarial, dihydrofolate reductase inhibitor.
Storage
Store below 30°C in a dry place, away from light and out of the reach of children.
Prepared by the Oshud Alo team from standard drug references. For information only; always follow your doctor's advice.