How cotrimoxazole works
Cotrimoxazole is a combination preparation of the antibiotics sulfamethoxazole and trimethoprim. Both substances inhibit the formation of folic acid in certain bacteria and fungi. This is needed for the synthesis of some building blocks of genetic material (thymidine and purines). Cotrimoxazole interrupts folic acid synthesis in two different ways:
- Trimethoprim inhibits the enzyme dihydrofolic acid reductase, which converts the folic acid precursor into the end product, tetrahydrofolic acid. This also stops the formation of the DNA building blocks and prevents the bacteria from multiplying.
Absorption, degradation and excretion
After ingestion by mouth (peroral), cotrimoxazole is largely absorbed into the blood via the intestinal mucosa. Maximum plasma concentrations of the two active ingredients are reached after about two to four hours.
Trimethoprim is excreted by the kidneys after about twelve hours, and half of sulfamethoxazole after about ten hours (half-lives). In the case of renal dysfunction, the half-lives are prolonged accordingly.
Cotrimoxazole is especially often used for bacterial urinary tract infections. It also helps with bacterial infections of the upper and lower respiratory tract, female and male genitalia, and gastrointestinal tract.
How cotrimoxazole is used
Cotrimoxazole may be prescribed as a juice, solution, or tablet with various concentrations of active ingredient.
For the treatment of pneumonia caused by fungi (Pneumocystis jiroveci) in patients with immunodeficiency, cotrimoxazole should be dosed four times higher.
The duration of therapy depends on the severity of the disease and the course of the disease. As a rule, treatment lasts five to eight days. It should always be taken after a meal.
What are the side effects of cotrimoxazole?
Occasional side effects include increased or decreased potassium in the blood, liver damage due to bile backing up (cholestatic hepatosis), and tinnitus (ringing in the ears).
When should cotrimoxazole not be taken?
Contraindications
Cotrimoxazole must not be used in:
- hypersensitivity to any of the active substances
- severe liver or kidney dysfunction
- abnormal blood count changes
- deficiency of the enzyme glucose-6-dehydrogenase
- osteomyelitis (bone marrow inflammation)
Interactions
Cotrimoxazole has a stronger effect than intended if medications that facilitate urination (e.g. probenecid) are taken at the same time. The same applies to concomitant use with non-steroidal anti-inflammatory drugs (NSAIDs, e.g., ibuprofen or acetylsalicylic acid).
Barbiturates (such as phenobarbital) and phenytoin (epilepsy drug) increase the likelihood of side effects in combination with cotrimoxazole.
Taking antibiotics such as cotrimoxazole can impair the effect of oral contraceptives (the pill). As a general rule, therefore, if you are taking the pill, you should also use a mechanical method of contraception, such as condoms, during antibiotic treatment and for seven days after that or until the end of the next pill break, to be on the safe side.
Age limit
Pregnancy and breastfeeding
Expectant mothers must not take the active ingredient during pregnancy, as it has not yet been adequately investigated how cotrimoxazole affects the unborn child. Although studies to date indicate no harmful effect, cotrimoxazole nevertheless remains a second-choice antibiotic in pregnancy.
How to obtain medicines containing cotrimoxazole
Cotrimoxazole is available in Germany, Austria and Switzerland as a juice, solution or in tablet form from pharmacies against a doctor’s prescription.
