pyrimethamine at a glance
Medicine class
Antiprotozoal and dihydrofolate-reductase inhibitor
Licensed use
Combination treatment of toxoplasmosis, including ocular, fetal and immune-deficiency-associated infection
Protective supplement
Calcium folinate is co-prescribed to reduce bone-marrow toxicity
Administration
Oral tablet swallowed with water
What is pyrimethamine used for?
The selected product is licensed for toxoplasmosis and must be used with a suitable synergistic anti-infective.
Toxoplasmosis affecting the eye
Confirmed fetal infection following maternal toxoplasmosis
Toxoplasmosis in people with impaired immunity, including infection affecting the brain or other organs
How does pyrimethamine work?
Pyrimethamine blocks the parasite enzyme dihydrofolate reductase. This limits production of reduced folates needed for parasite growth and replication. It has greater affinity for the protozoal enzyme than the human enzyme, although human folate metabolism and blood-cell production can still be affected.
How to take pyrimethamine
Take pyrimethamine exactly as prescribed and swallow the tablet with water. For toxoplasmosis it is prescribed with another suitable anti-infective and calcium folinate; do not omit either accompanying medicine unless the treating specialist changes the plan.
The tablets are supplied in child-resistant peel-and-push blister packaging.
Check with the prescriber or pharmacist before changing or stopping treatment.
pyrimethamine side effects
Common or expected effects
- Nausea, vomiting or diarrhoea
- Headache or dizziness
- Rash
- Anaemia, reduced white blood cells or reduced platelets
Get urgent medical advice
- Bone-marrow suppression causing infection, unusual bleeding, severe tiredness or breathlessness
- Seizure
- Severe skin reaction or breathing difficulty
- Pancytopenia, where several blood-cell types become severely reduced
pyrimethamine in pregnancy
Pyrimethamine is generally avoided during the first trimester because animal studies and its antifolate action raise concern about fetal harm. Specialist-led use later in pregnancy may be considered for confirmed placental or fetal infection, or serious maternal risk. Calcium folinate must accompany treatment. Discuss pregnancy or plans for pregnancy with the prescriber.
pyrimethamine while breastfeeding
Pyrimethamine enters breast milk. Product information advises avoiding breastfeeding during toxoplasmosis treatment because infant exposure may be substantial and other anti-infectives are given concurrently. Discuss feeding options with the specialist, midwife or pharmacist.
pyrimethamine interactions
Review the complete medicine list before treatment, particularly medicines affecting folate or bone-marrow function.
Trimethoprim or co-trimoxazole
Combined folate inhibition can increase the risk of megaloblastic anaemia and other blood-cell abnormalities.
Proguanil
Its antifolate effect may add to pyrimethamine-associated folate and bone-marrow toxicity.
Methotrexate and other cytotoxic medicines
The combination may increase bone-marrow suppression; seizures have also been reported with methotrexate in children.
Zidovudine
Additive bone-marrow suppression may increase the risk of anaemia or other low blood counts.
Common questions about pyrimethamine
Answers are fully visible for fast scanning and source review.
What is pyrimethamine used for?
The selected product is licensed for combination treatment of toxoplasmosis, including ocular, fetal and immune-deficiency-associated infection.
Is pyrimethamine an antibiotic?
It is an antiprotozoal rather than a conventional antibacterial antibiotic. It acts against parasites such as Toxoplasma gondii.
Why is pyrimethamine combined with another anti-infective?
Sources
These sources were used to prepare and review this medicine page.
Prepared and reviewed by the iatroX editorial team.