Side effects of trimethoprim

Updated February 12, 2018

Trimethoprim is an antibiotic prescribed to treat urinary tract infections, traveller's diarrhoea, respiratory infections and ear infections. It is sold in generic form and under brand names of Primsol, Proloprim and Trimpex. Several side effects are possible.

Most Common Side Effects

The most common side effects linked to trimethoprim include loss of appetite, altered taste, abdominal pain, nausea, vomiting and diarrhoea. Taking the medication with food can decrease some of these effects. Headaches are also possible.


Taking trimethoprim can result in anaemia. Cases usually are mild and resolve when the medication is discontinued. Symptoms include white or bluish fingernails, unusual tiredness and weakness.

Additional Blood Effects

Other blood-related side effects of trimethoprim can include low platelet counts and low white cell counts.

Skin Side Effects

Skin-related side effects associated with trimethoprim include rash and itching. The rash can eventually blister and peel, although this is rare. Some people develop the rash only on areas of the skin exposed to sunlight.

Pregnancy and Breastfeeding

Research with animals indicates that trimethoprim has negative effects on the foetus. It also is released into breast milk in high concentrations, so breastfeeding women should avoid taking this medication.

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About the Author

Shelley Moore is a journalist and award-winning short-story writer. She specializes in writing about personal development, health, careers and personal finance. Moore has been published in "Family Circle" magazine and the "Milwaukee Sentinel" newspaper, along with numerous other national and regional magazines, daily and weekly newspapers and corporate publications. She has a Bachelor of Science in psychology.