Pharmacology of Co-trimoxazole


Co-trimoxazole is a combination antibiotic that includes two active ingredients: sulfamethoxazole and trimethoprim. These two components work synergistically to enhance their antibacterial effectiveness. Below is a comprehensive overview of the pharmacology of co-trimoxazole:

Mechanism of Action

Co-trimoxazole works by inhibiting two consecutive steps (sequential blockade) in the biosynthesis of nucleic acids and proteins essential to bacteria. Sulfamethoxazole inhibits the synthesis of dihydrofolic acid, while trimethoprim inhibits the production of tetrahydrofolic acid. By targeting these two steps, co-trimoxazole effectively halts bacterial growth and replication.



  • Absorption: Both components of co-trimoxazole are well absorbed from the gastrointestinal tract.
  • Distribution: They are widely distributed throughout the body, including in tissues and body fluids.
  • Metabolism: Sulfamethoxazole is partially metabolized in the liver, while trimethoprim is mostly excreted unchanged.
  • Excretion: Both drugs are excreted primarily through the kidneys.

Adverse Effects

  • Gastrointestinal Disturbances: Nausea, vomiting, and diarrhea.
  • Skin Reactions: Rashes, and in rare cases, severe skin reactions like Stevens-Johnson syndrome.
  • Blood Disorders: Such as anemia, leukopenia, and thrombocytopenia, particularly in patients with a deficiency in the enzyme glucose-6-phosphate dehydrogenase (G6PD).
  • Hepatotoxicity: Liver enzyme abnormalities and, in rare cases, hepatitis.
  • Hyperkalemia: Elevated levels of potassium in the blood, particularly in elderly patients or those with kidney dysfunction.

Clinical Uses

  • Urinary Tract Infections: One of the most common uses of co-trimoxazole.
Cotrimoxazole is used in URI
#Urinary tract infection or UTI vector illustration Labeled medical scheme
  • Respiratory Tract Infections: Including bronchitis and pneumonia caused by susceptible strains of bacteria.
#Respiratory Tract Infection
  • Gastrointestinal Infections: Such as shigellosis and traveler’s diarrhea.
  • Skin and Soft Tissue Infections
  • Prevention and Treatment of Pneumocystis jirovecii Pneumonia: Particularly in immunocompromised patients, such as those with HIV/AIDS.


Bacterial resistance to co-trimoxazole can occur due to mutations that alter the target enzymes, decreased uptake of the drug, or increased production of p-aminobenzoic acid (a substrate for dihydropteroate synthase).

Drug Interactions

  • Warfarin: Co-trimoxazole can enhance the anticoagulant effects of warfarin.
  • Methotrexate: Co-trimoxazole can increase the toxicity of methotrexate.
  • Phenytoin: Co-trimoxazole can increase the levels of phenytoin in the blood, potentially leading to toxicity.
  • Sulfonylureas: Co-trimoxazole can enhance the hypoglycemic effects of sulfonylureas.


  • Allergy to Sulfonamides or Trimethoprim: Patients with a history of allergy to any component of co-trimoxazole should not receive this medication.
  • Pregnancy and Breastfeeding: Co-trimoxazole can interfere with folic acid metabolism, and its use is generally avoided during pregnancy and breastfeeding.
  • Severe Kidney or Liver Disease: Use with caution and close monitoring.


Co-trimoxazole is a combination antibiotic with a broad spectrum of activity. It is effective against a variety of bacterial infections, including urinary tract infections, respiratory tract infections, and certain types of pneumonia. However, its use must be carefully considered due to the potential for adverse effects and the development of bacterial resistance. Monitoring for drug interactions and contraindications is crucial for safe and effective use.

Disclaimer: This article is for informational purposes only and does not constitute medical advice. Always seek the advice of a healthcare provider with any questions regarding a medical condition.

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