Idotrim (Trimethoprim)

Trimethoprim is an antibacterial that interferes with bacterial folic acid synthesis, making it effective for urinary tract infections.

• Urinary tract infection: reduces bacterial load and resolves symptoms such as dysuria and frequency.
• Acute uncomplicated cystitis: clears infection in the bladder, alleviating pain and urinary urgency.
• Prophylaxis of recurrent urinary infections: maintains low bacterial counts to prevent repeat episodes.

• Trimethoprim selectively inhibits bacterial dihydrofolate reductase, an enzyme required for the reduction of dihydrofolic acid to tetrahydrofolic acid. By blocking this step, the drug prevents synthesis of nucleic acids and proteins essential for bacterial growth and replication, leading to a bacteriostatic effect against susceptible Gram‑negative and some Gram‑positive organisms. The inhibition is competitive with the substrate and does not affect human dihydrofolate reductase at therapeutic concentrations.

Trimethoprim may cause a range of adverse reactions, varying from mild to serious.

Common Side Effects

• Nausea: mild stomach upset often transient.
• Vomiting: occasional emesis.
• Diarrhea: loose stools.
• Skin rash: mild erythematous eruption.
• Elevated serum creatinine: reversible increase in kidney function tests.
• Headache: mild to moderate tension‑type pain.

Serious or Rare Side Effects

• Severe allergic reaction: anaphylaxis or angioedema requiring immediate medical attention.
• Blood dyscrasias: agranulocytosis, aplastic anemia, or leukopenia.
• Hepatotoxicity: elevated liver enzymes or hepatitis.
• Stevens‑Johnson syndrome: severe mucocutaneous reaction.
• Hemolytic anemia in G6PD deficiency: rapid destruction of red blood cells.
• Hyperkalemia: elevated potassium levels, especially with concomitant drugs.

When prescribing Trimethoprim, clinicians should consider several precautionary measures to ensure safe use.

• Renal impairment: adjust dose or monitor renal function in patients with reduced kidney clearance.
• Hepatic dysfunction: use cautiously in patients with liver disease and monitor liver enzymes.
• Pregnancy: evaluate risk‑benefit, as safety data are limited; avoid in first trimester unless necessary.
• Concomitant folate antagonists: avoid using with methotrexate or pyrimethamine due to additive folate inhibition.
• Drug‑induced hyperkalemia: monitor potassium levels when combined with potassium‑sparing diuretics.
• Store at ambient temperature: keep below 25 °C, protect from moisture and light in the original packaging.

Trimethoprim can interact with several medicines, altering its efficacy or increasing toxicity.

Major Interactions (Avoid)

• Methotrexate: increased toxicity due to additive folate antagonism.
• Phenytoin: reduced plasma levels of trimethoprim, decreasing effectiveness.
• Sulfonamides: heightened risk of severe skin reactions and bone marrow suppression.

Moderate Interactions (Monitor Closely)

• Loop diuretics (e.g., furosemide): risk of hyperkalemia.
• ACE inhibitors or ARBs: potential for elevated potassium.
• Oral anticoagulants (warfarin): may enhance anticoagulant effect, requiring INR monitoring.
• NSAIDs: may impair renal function when combined.