Pyrimethamine Side Effects

Medically reviewed by Drugs.com. Last updated on Jun 1, 2025.

Applies to pyrimethamine: oral tablet.

Precautions

It is very important that your doctor check your or your child's progress at regular visits to make sure that this medicine is working properly. Blood tests are needed to check for unwanted effects.

If your symptoms do not improve within a few days or if they become worse, check with your doctor.

This medicine may decrease the amount of folate in the body. Tell your doctor if you have burning, crawling, itching, numbness, prickling, "pins and needles", or tingling feelings, changes in vision, confusion, depression, redness or soreness of the tongue, ulcers inside the mouth, unusual tiredness or weakness.

Do not drink alcohol while you are using this medicine.

Do not take other medicines unless they have been discussed with your doctor. This includes prescription or nonprescription (over-the-counter [OTC]) medicines and herbal or vitamin supplements.

Serious side effects of pyrimethamine

Along with its needed effects, pyrimethamine may cause some unwanted effects. Although not all of these side effects may occur, if they do occur they may need medical attention.

Check with your doctor immediately if any of the following side effects occur while taking pyrimethamine:

Other side effects of pyrimethamine

Some side effects of pyrimethamine may occur that usually do not need medical attention. These side effects may go away during treatment as your body adjusts to the medicine. Also, your health care professional may be able to tell you about ways to prevent or reduce some of these side effects.

Check with your health care professional if any of the following side effects continue or are bothersome or if you have any questions about them:

See also:

For healthcare professionals

Applies to pyrimethamine: compounding powder, oral tablet.

General adverse events

Since this drug is coadministered with a sulfonamide, the manufacturer product information for the relevant sulfonamide should be consulted for sulfonamide-related side effects.^[Ref]

Gastrointestinal

• Very common (10% or more): Nausea, vomiting, diarrhea, colic
• Uncommon (0.1% to 1%): Dry mouth, dry throat
• Very rare (less than 0.01%): Buccal ulceration
• Frequency not reported: Glossitis, atrophic glossitis^[Ref]

Nervous system

• Very common (10% or more): Headache
• Common (1% to 10%): Dizziness
• Very rare (less than 0.01%): Convulsions/seizures
• Frequency not reported: Ataxia, tremors, precipitation of grand mal attack^[Ref]

Precipitation of a grand mal attack in a patient predisposed to epilepsy was reported; clinical significance was not established.^[Ref]

Hematologic

• Very common (10% or more): Anemia
• Common (1% to 10%): Thrombocytopenia, leukopenia
• Very rare (less than 0.01%): Pancytopenia
• Frequency not reported: Megaloblastic anemia, neutropenia, agranulocytosis^[Ref]

Dermatologic

• Very common (10% or more): Rash
• Uncommon (0.1% to 1%): Abnormal skin pigmentation, dermatitis

Other

• Uncommon (0.1% to 1%): Fever, malaise

Psychiatric

• Uncommon (0.1% to 1%): Depression
• Very rare (less than 0.01%): Insomnia

Insomnia was reported when this drug was given weekly at higher than recommended doses.

Respiratory

• Rare (0.01% to 0.1%): Pulmonary eosinophilia
• Very rare (less than 0.01%): Pneumonia with cellular and eosinophilic pulmonary infiltration^[Ref]

Pneumonia with cellular and eosinophilic pulmonary infiltration was observed when this drug was administered once a week with sulfadoxine.^[Ref]

Hypersensitivity

• Frequency not reported: Hypersensitivity reactions (included erythema multiforme, Stevens-Johnson Syndrome, toxic epidermal necrolysis, Lyell's syndrome, hepatitis, anaphylaxis, pulmonary reactions, maculopapular rashes)^[Ref]

Cardiovascular

• Very rare (less than 0.01%): Circulatory collapse
• Frequency not reported: Disorders of cardiac rhythm, arrhythmias^[Ref]

Circulatory collapse was reported in patients treated with higher than recommended doses.^[Ref]

Metabolic

• Very rare (less than 0.01%): Hyperphenylalaninemia
• Frequency not reported: Anorexia^[Ref]

Hyperphenylalaninemia was reported in neonates treated for congenital toxoplasmosis.^[Ref]

Genitourinary

• Very rare (less than 0.01%): Hematuria

Oncologic

• Frequency not reported: Chronic granulocytic leukemia, reticulum cell sarcoma^[Ref]

A 51-year-old female developed chronic granulocytic leukemia after taking this drug for 2 years for toxoplasmosis and a 56-year-old patient developed reticulum cell sarcoma after taking this drug for 14 months for toxoplasmosis.^[Ref]

Further information

Pyrimethamine side effects can vary depending on the individual. Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.

Note: Medication side effects may be underreported. If you are experiencing side effects that are not listed, submit a report to the FDA by following this guide.

Medical Disclaimer