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Trimethoprim is an antibiotic from the group of diaminopyrimidines.

Trimethoprim is an antibiotic. This medicine prevents protein production in bacteria, causing them to die. The active substance is absorbed into the blood and reaches the inflamed tissues and the urine in this way. Trimethoprim is only available with a medical prescription.


Trimethoprim has the sum formula C14H18N4O3 and a molar mass of 290.32 g / mol.

Precautions of Trimethoprim

Some medicines are not suitable for people with certain conditions, and sometimes a medicine can only be used if extra care is taken. For these reasons, before you start taking trimethoprim it is important that your doctor knows:

  • If you are pregnant, trying for a baby or breast-feeding.
  • If you know you have low amounts of the vitamin folic acid.
  • If you have problems with the way your kidneys work.
  • If you have a blood disorder, or if you have a rare inherited blood disorder called porphyria.
  • If you are taking any other medicines. This includes any medicines which are available to buy without a prescription, as well as herbal and complementary medicines.
  • If you have ever had an allergic reaction to a medicine.


Trimethoprim is available in various starches. The doctor determines which strength best suits your symptoms. If the doctor does not prescribe otherwise, the generally recommended dosage of trimethoprim is as follows:

Adults and children from 12 years of age: Twice daily 200mg. In case of a bladder infection, the doctor may advise you to take a daily dose of 300mg in the evening.

For children younger than 12 years of age, a different recommendation for dosage applies. Please read the recommended dosage and read the package insert before use.

Mechanism of action

Trimethoprim inhibits the bacterial dihydrofolate reductase. This converts dihydrofolic acid into tetrahydrofolic acid. Tetrahydrofolic acid is an important substrate in the production of purines and thymidine in bacterial cells.

If these DNA building blocks are absent, growth inhibition of the bacterial cells occurs. The effect of trimethoprim on human dihydrofolate reductase is generally low at therapeutic doses.

Effect spectrum

Trimethoprim works against a broad spectrum of gramnegative and gram-positive organisms. In the case of monotherapy with trimethoprim, however, the development of antibiotic resistance frequently occurs rapidly.

Side effects

Trimethoprim is generally well tolerated. Occasionally, unspecific gastrointestinal symptoms and itchy exanthema (eruption of the drug) occur.

There is rarely a relevant inhibition of the human dihydrofolate reductase with a following deficiency of tetrahydrofolic acid for the human organism. This results in a disorder of hematopoiesis with the formation of anemia, leukopenia and / or thrombopenia.

Trimethoprim and Alcohol

Trimethoprim does not interact with alcohol, which means that you can drink alcohol during your treatment. However, if you are taking trimethoprim for a UTI (urinary tract infection), you should avoid alcohol and ensure you drink plenty of clear fluids.


Trimethoprim is rarely used as a monotherapy. They are usually used as a co-trimoxazole in a fixed combination with the sulfonamide antibiotic sulfamethoxazole. In this context, it is used to treat urinary tract infections and respiratory infections. Pneumocystis pneumonia is a special indication, in which cotrimoxazole has to be used for a long time.

As a monotherapy, trimethoprim can be used for the treatment of uncomplicated cystitis and for recurrent prophylaxis of urinary tract infections in (small) children. However, the application as monotherapy is controversial.

When should I take this medicine?

Trimethoprim is not a suitable medicine for every patient. Do not take this medicine if:

  • Allergic to trimethoprim or any of the other ingredients;
  • Severe liver or kidney disease;
  • To suffer from a blood disease.

If you have other health problems or if you are taking other medicines, please ask your doctor if you are taking Trimethoprim safely.

Pregnancy / Drug Abuse / Alcohol

If you are pregnant or breast-feeding, do not take Trimethoprim, or only on the advice of your doctor.

This medicine does not affect your ability to drive.

There are no indications / warnings regarding the consumption of alcohol.

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