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Trimethoprim: effect, areas of application, side effects

by Josephine Andrews
Published: Last Updated on 426 views

Trimethoprim is an antibiotic that interferes with the folic acid metabolism of bacteria. As a result, the germs can no longer divide and multiply – their growth is inhibited. Trimethoprim is therefore called a bacteriostatic agent. Here you can read everything interesting about the effects and use of trimethoprim as well as possible side effects and interactions.

This is how trimethoprim works

Trimethoprim is a bacterial growth-inhibiting (bacteriostatic) antibiotic from the chemical group of diaminobenzylpyrimidines. The active ingredient is often combined with sulfamethoxazole – the combination increases the antibacterial effect and is known as cotrimoxazole.

Both humans and microorganisms such as fungi and bacteria need folic acid (vitamin B9) as a component of enzymes (as a so-called cofactor) that are necessary to regenerate the building blocks of the genetic material (DNA). Since the genetic material has to double with every cell division and thus with every type of growth, folic acid has a vital task.

However, in order to be used by the cells, the folic acid in the body must first be activated. To do this, it is converted twice by the enzyme DHFR (dihydrofolate reductase), which produces first DHF (dihydrofolate) and then THF (tetrahydrofolate). THF is the active form of the vitamin and can be used for metabolic processes.

Since the bacterial DHFR enzyme clearly decides from the human DHFR enzyme, it can be specifically inhibited with suitable drugs such as trimethoprim. As a result, the bacteria are no longer able to duplicate their genetic material – they can no longer divide.

DHFR inhibitors such as trimethoprim therefore have a bacteriostatic effect: They do not kill the bacteria, but only inhibit their growth. This gives the body’s immune system a chance to get a bacterial infection under control.

intake, degradation and excretion

After ingestion, the active ingredient is quickly and completely absorbed into the bloodstream via the intestinal mucosa. The highest concentrations are reached there after about one to four hours. The antibiotic reaches the site of infection via the blood.

About a fifth of the dose taken is broken down by the liver, the rest is excreted unchanged via the kidneys with the urine, which means that the active ingredient is also suitable for the treatment of urinary tract infections. Eight to 14 hours after ingestion, half of the active ingredient has left the body again.

When is trimethoprim used?

Trimethoprim as a single active ingredient is only used very rarely in bacterial infections, since bacterial resistance can quickly develop (that is, bacteria become insensitive to the active ingredient). Instead, a combination of trimethoprim and sulfamethoxazole is usually prescribed, which is called cotrimoxazole in a fixed 1:5 composition.

Trimethoprim as the sole antibiotic is approved for the short-term treatment of uncomplicated urinary tract infections and for the long-term prevention of recurrent urinary tract infections. Other approved indications are infections of the digestive or respiratory tract caused by pathogens sensitive to trimethoprim.

How trimethoprim is used

Trimethoprim can be taken in pill form or as a juice. The juice is particularly suitable for children under the age of six, as they usually still have difficulty swallowing tablets.

For the treatment of acute bacterial infections, children over the age of twelve and adults take 150 to 200 milligrams of trimethoprim twice a day with a glass of water, regardless of meals. In younger children, the dosage is usually adjusted to body weight. The duration of therapy for acute infections usually extends over five to ten days.

For long-term prevention over six weeks to half a year, 100 milligrams of trimethoprim are taken once a day in the evening.

What are the side effects of trimethoprim?

One in ten to a hundred people treated show side effects such as loss of appetite, taste disturbances, pain in the upper abdomen, nausea, vomiting, diarrhea, skin rashes and/or inflammation of the tongue and gums.

If severe and prolonged diarrhea occurs during or shortly after antibiotic therapy, a doctor should be consulted. It could be an antibiotic-related bowel disease.

Fever, changes in blood values ​​and infections with fungi or trimethoprim-resistant bacteria are occasionally observed – the latter especially with long-term use of the antibiotic.

Trimethoprim can make the skin more sensitive to light. You should therefore avoid intensive exposure to the sun during the treatment.

Possible discoloration of teeth caused by antibiotic therapy can be prevented by intensive oral hygiene.

What should be considered when taking trimethoprim?


Trimethoprim must not be used in:

  • Damage to the blood-forming system such as a lack of blood platelets (thrombocytopenia) or a lack of white blood cells (leukocytopenia)
  • severe liver or kidney dysfunction
  • Fragile X chromosome (hereditary disease) linked to mental retardation in children


Trimethoprim is partially metabolized in the liver by the cytochrome P450 2C8 enzyme system. If other drugs that are also broken down by this enzyme are given at the same time, they can accumulate. These include, for example, the blood sugar reducers repaglinide and rosiglitazone (medicines for diabetes), the diuretic torasemide and the strong painkiller buprenorphine .

The effectiveness of anticoagulants such as warfarin may be increased by trimethoprim, so coagulation values ​​should be monitored closely.

Taking trimethoprim in combination with certain high blood pressure medicines can increase blood potassium levels. Examples of such drugs include ACE inhibitors (such as enalapril, ramipril), sartans (such as valsartan , candesartan ), and potassium-sparing diuretics (such as amiloride, triamterene , spironolactone).

The effect of the pill (for hormonal contraception) can be reduced by trimethoprim. Women should therefore also use non-hormonal contraception (e.g. with a condom) during antibiotic treatment.

age restriction

Adjusted doses of trimethoprim have been approved for infants as young as six weeks.

pregnancy and breast feeding period

Trimethoprim is able to cross the placenta and enters breast milk. There are better-studied and better-tolerated antibiotics available for pregnant and lactating women that are preferable (such as penicillins, cephalosporins, and macrolides). However, after a strict medical benefit-risk assessment, the use of trimethoprim during pregnancy and breastfeeding is possible.

How to get medication with trimethoprim

Medicines containing trimethoprim are only available in pharmacies in Germany, Austria and Switzerland with a prescription from a doctor.

Since when is trimethoprim known?

Trimethoprim was first approved for use in humans in 1962, followed six years later by the combination cotrimoxazole, which is still used today. Both antibiotics are now sold as generics, with many manufacturers offering the combination in particular. On the other hand, preparations that only contain the active ingredient trimethoprim alone have become rare

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