BMA Concise Guide to Medicine & Drugs


Brand names None

Used in the following combined preparations Polytrim, Septrin


Drug group Antibacterial drug

Overdose danger rating Low

Dependence rating Low

Prescription needed Yes

Available as generic Yes


Trimethoprim is an antibacterial drug that became popular in the 1970s for prevention and treatment of infections of the urinary and respiratory tracts. The drug has been used for many years in combination with another antibacterial, sulfamethoxazole, in a preparation known as co-trimoxazole. Trimethoprim, however, has fewer adverse effects than co-trimoxazole and is equally effective in treating many conditions.

Although side effects of trimethoprim are not usually troublesome, tests to monitor blood composition are often advised when the drug is taken for prolonged periods.


Your drug prescription is tailored for you. Do not alter dosage without checking with your doctor.

How taken/used Tablets, liquid, injection.

Frequency and timing of doses 1–2 x daily.

Adult dosage range 400mg daily (treatment); 100mg daily (prevention).

Onset of effect 1–4 hours.

Duration of action Up to 24 hours.

Diet advice None.

Storage Keep in original container at room temperature out of the reach of children. Protect from light.

Missed dose Take as soon as you remember.

Stopping the drug Take the full course. Even if you feel better, the original infection may still be present and symptoms may recur if treatment is stopped too soon.

Exceeding the dose An occasional unintentional extra dose is unlikely to be a cause for concern. But if you notice any unusual symptoms, or if a large overdose has been taken, notify your doctor.


Trimethorpim on its own rarely causes side effects but additional adverse effects may occur when it is taken as the combined preparation co-trimoxazole (consisting of trimethoprim and sulfamethoxazole). The adverse effects covered here are for trimethoprim alone; for those of co-trimoxazole. The adverse effects of trimethoprim include nausea, vomiting, rash, itching, sore throat, fever, spontaneous bleeding, and easy bruising. Discuss with your doctor if nausea and/or vomiting are severe. If any of the other side effects occur, stop taking the drug and contact your doctor promptly.


Cytotoxic drugs Trimethoprim increases the risk of blood problems if taken with azathioprine or mercaptopurine. Taken with methotrexate, there is an increased risk of folate deficiency.

Ciclosporin Trimethoprim increases the risk of this drug causing kidney damage.

Phenytoin Taken with trimethoprim, this drug may increase the risk of folic acid deficiency, resulting in blood abnormalities.

Warfarin Trimethoprim may increase the anticoagulant effect of warfarin.

Antimalarials containing pyrimethamine Drugs such as fansidar or maloprim may increase the risk of folic acid deficiency, resulting in blood abnormalities, if they are taken with trimethoprim.

ACE inhibitors and angiotensin II blockers Trimethoprim increases the risk of high potassium levels in the blood when used with these drugs.


Be sure to tell your doctor if:

· You have long-term liver or kidney problems.

· You have a blood disorder.

· You have porphyria.

· You are taking other medicines.

Pregnancy Not prescribed. May cause defects in the baby.

Breast-feeding The drug passes into the breast milk, but at normal doses adverse effects on the baby are unlikely. Discuss with your doctor.

Infants and children Reduced dose necessary.

Over 60 Increased likelihood of adverse effects. Reduced dose may be required.

Driving and hazardous work No known problems.

Alcohol No known problems.


Long-term use of this drug may lead to folate deficiency, which, in turn, may lead to blood abnormalities. Folate supplements may be prescribed.

Monitoring Periodic blood tests to monitor blood composition are usually advised.