Monotrim (Trimethoprim)

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

• Urinary tract infection: eradicates susceptible bacteria, relieving symptoms and preventing complications.
• Acute uncomplicated cystitis: provides rapid bacterial clearance, leading to symptom resolution in most patients.
• Prophylaxis of recurrent urinary tract infection: reduces the incidence of future infections when taken as prescribed.

• Trimethoprim selectively inhibits the bacterial enzyme dihydrofolate reductase, which is essential for the reduction of dihydrofolic acid to tetrahydrofolic acid. By blocking this step, the drug prevents synthesis of nucleic acids and proteins required for bacterial replication, resulting in a bacteriostatic effect against susceptible organisms. The inhibition is competitive with the substrate and is reversible, allowing normal human dihydrofolate reductase, which has lower affinity, to remain largely unaffected, thereby limiting toxicity to host cells.

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

Common Side Effects

• Nausea: mild stomach upset that may resolve without treatment.
• Vomiting: occasional episodes, especially with higher doses.
• Diarrhea: loose stools that are usually self‑limiting.
• Skin rash: mild erythema or itching.
• Elevated serum creatinine: transient rise in kidney function tests.
• Hyperkalemia: modest increase in blood potassium levels.

Serious or Rare Side Effects

• Severe allergic reaction: anaphylaxis or angioedema requiring emergency care.
• Hematologic abnormalities: aplastic anemia, leukopenia, or thrombocytopenia.
• Stevens‑Johnson syndrome: severe skin and mucous membrane involvement.
• Hepatotoxicity: liver enzyme elevation or hepatitis.
• Renal toxicity: acute interstitial nephritis.

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

• Renal impairment: adjust dose or monitor renal function closely.
• Hepatic dysfunction: use with caution in patients with liver disease.
• Concomitant folate antagonists: avoid or monitor for additive bone‑marrow suppression.
• Pregnancy: assess risk‑benefit as safety data are limited.
• Elderly patients: start at lower dose and monitor for adverse effects.
• Store at ambient temperature: keep below 25 °C, protect from moisture and light in the original container.

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

Major Interactions (Avoid)

• Methotrexate: increased risk of severe myelosuppression.
• Phenytoin: reduced plasma levels of Trimethoprim, decreasing effectiveness.
• Cyclosporine: heightened nephrotoxicity risk.

Moderate Interactions (Monitor Closely)

• Warfarin: potentiation of anticoagulant effect, requiring INR monitoring.
• Digoxin: possible increase in serum digoxin concentrations.
• Oral contraceptives: may reduce contraceptive efficacy.
• Sulfonamides: additive folate antagonism may enhance bone‑marrow suppression.