Home Medicines Co-trimoxazole: effect, areas of application, side effects

Co-trimoxazole: effect, areas of application, side effects

by Josephine Andrews
Published: Last Updated on 292 views

Co- trimoxazole is a combination of two proven antibiotics from the group of folic acid antagonists: trimethoprim and sulfamethoxazole. It helps particularly well with infections of the urinary tract, gastrointestinal tract, upper and lower respiratory tract. Read everything you need to know about the mode of action, side effects and use of cotrimoxazole here.

How does cotrimoxazole work?

Co-trimoxazole is a combination of the antibiotics sulfamethoxazole and trimethoprim. Both substances inhibit the formation of folic acid in certain bacteria and fungi. This is required for the synthesis of some building blocks of the genetic material (thymidine and purines). Co-trimoxazole disrupts folic acid synthesis in two different ways:

  • Sulfamethoxazole prevents the formation of a folic acid precursor (dihydrofolic acid) by binding to the enzyme responsible for this (dihydropteroic acid synthetase) and overriding it. Since this mechanism is reversible, however, constantly high sulfonamide concentrations are required.
  • Trimethoprim inhibits the enzyme dihydrofolic acid reductase, which converts the folic acid precursor into the end product, tetrahydrofolic acid. Here, too, the formation of the DNA building blocks is stopped and the multiplication of the bacteria is prevented.

Co-trimoxazole works best when the two active ingredients are present in the body in a concentration ratio of 5:1 (sulfamethoxazole:trimethoprim). The mutually supporting effects of the two substances increase the effectiveness, expand the spectrum of action and delay the development of bacterial resistance to the antibiotic.

intake, degradation and excretion

After taking it by mouth (orally), cotrimoxazole is mostly absorbed into the blood via the intestinal mucosa. Maximum plasma concentrations of the two active ingredients contained are reached after about two to four hours.

Half of the trimethoprim is excreted via the kidneys after around twelve hours, and half of the sulfamethoxazole after around ten hours (half-lives). In the case of renal dysfunction, the half-lives increase accordingly.

When is co-trimoxazole used?

Co-trimoxazole is used particularly often in bacterial urinary tract infections. It also helps with bacterial infections of the upper and lower respiratory tract, female and male genitals, and the gastrointestinal tract.

How Co-trimoxazole is used

Co-trimoxazole can be prescribed as a juice, solution, or tablet with various concentrations of the active ingredient.

For adults and adolescents over the age of 13, the average dose is 720 to 960 milligrams of the active ingredient combination per day. Children from six to twelve years should take one tablet with 480 milligrams of cotrimoxazole twice a day.

For the treatment of pneumonia caused by fungi (Pneumocystis jiroveci) in patients with immunodeficiency, the dose of cotrimoxazole should be four times higher.

The duration of therapy depends on the severity of the disease and the course of the disease. The treatment usually lasts five to eight days. It should always be taken after a meal.

What are the side effects of cotrimoxazole?

Common side effects are allergic skin reactions , which can manifest as itching (pruritus) or a rash (exanthema). Symptoms of the gastrointestinal tract such as pain in the upper abdomen, loss of appetite, nausea, vomiting and diarrhea are also possible.

Occasionally occurring side effects are increased or decreased potassium levels in the blood, liver damage due to backlog of bile (cholestatic hepatosis) and tinnitus (ringing in the ears).

Very rarely, a reduced number of blood platelets (thrombocytoes) and neutrophils (a subgroup of white blood cells) and an increase in liver enzymes in the blood develop after the use of cotrimoxazole. These side effects are increasingly found in patients over 60 years of age.

When not to take Co-trimoxazole?


Co-trimoxazole must not be used with:

  • Hypersensitivity to any of the active ingredients
  • severe hepatic or renal impairment
  • abnormal blood count changes
  • Deficiency of the enzyme glucose-6-dehydrogenase
  • Osteomyelitis (bone marrow inflammation)


Local anesthetics (such as benzocaine), gastric acid-regulating substances (mineral antacids) and the sedative paraldehyde can reduce the effect of cotrimoxazole. A simultaneous intake of such active substances with the antibiotic should therefore be avoided.

Co-trimoxazole works more than intended when taken at the same time as medications that make urination easier (eg probenecid). The same applies to simultaneous use with non-steroidal anti-inflammatory drugs (NSAIDs, eg ibuprofen or acetylsalicylic acid).

Barbiturates (such as phenobarbital) and phenytoin (epilepsy drug) increase the likelihood of side effects when combined with cotrimoxazole.

Other medications that also interfere with folic acid metabolism (such as methotrexate) increase the risk of folic acid deficiency states when used at the same time.

Taking antibiotics such as co-trimoxazole can impair the effectiveness of oral contraceptives (the pill ). In general, therefore, the following applies: If you take the pill, you should also use a mechanical contraceptive method, such as condoms , to be on the safe side during antibiotic treatment and for seven days afterwards or until after the end of the next pill break .

age restriction

Co-trimoxazole can be used in infants from the 6th week of life in an appropriately adjusted dosage.

pregnancy and breast feeding period

Expectant mothers should not take the active ingredient during pregnancy, as there has not yet been sufficient research into how cotrimoxazole affects the unborn child. Studies to date speak against a damaging effect, but cotrimoxazole remains a second-choice antibiotic in pregnancy.

The two active ingredients in cotrimoxazole are only slightly excreted in breast milk. No significant symptoms are to be expected in the breastfed child if the mother is treated with the antibiotic. However, since the data situation does not allow a definitive assessment here either, cotrimoxazole is also only a second-choice antibiotic during breastfeeding.

How to get medicines with cotrimoxazole

Co-trimoxazole is available in Germany, Austria and Switzerland as a juice, solution or in tablet form against a doctor’s prescription in pharmacies.

You may also like

Leave a Comment