Trimethoprim is an antibiotic belonging to the class of dihydrofolate reductase inhibitors, widely used for the prevention and treatment of bacterial infections, especially those of the urinary tract. Unlike broad-spectrum antibiotics, trimethoprim has a relatively targeted mechanism of action: it selectively inhibits bacterial dihydrofolate reductase, an enzyme critical to folic acid metabolism and DNA synthesis. By disrupting this pathway, it prevents bacteria from replicating and surviving. Human cells are less affected because they rely on different pathways for folate metabolism, allowing for a degree of selective toxicity. Trimethoprim is often combined with sulfamethoxazole (as co-trimoxazole), producing a synergistic effect that blocks sequential steps in folic acid synthesis and increases antibacterial potency.
Trimethoprim is widely used as a single agent, particularly in uncomplicated urinary tract infections (UTIs), where its narrow spectrum helps limit disruption of normal bacterial flora. It has good oral bioavailability, is rapidly absorbed, and reaches effective concentrations in the urine, kidneys, and prostate. This makes it a first-line or alternative choice in many treatment guidelines for UTIs, though resistance rates vary globally and must be considered in prescribing decisions.
Beyond its use in UTIs, trimethoprim is used in prophylaxis for recurrent infections and in certain immunocompromised patients, such as those at risk of Pneumocystis jirovecii pneumonia (PJP), though usually in combination therapy. Its relatively safe profile, convenient oral dosing, and long history of use make it a key medicine in infectious disease practice, though rising resistance remains a growing challenge in its long-term utility.
Trimethoprim is indicated for the treatment and prevention of bacterial infections where susceptible organisms are present:
Adherence is important to prevent incomplete eradication of bacteria and to reduce risk of resistance. Patients should be advised to complete the prescribed course, even if symptoms improve early.
Like all antibiotics, trimethoprim can cause adverse effects, ranging from mild to rare but severe reactions:
Patients should be advised to seek immediate medical attention if they experience symptoms such as persistent fever, sore throat, unexplained bruising/bleeding, or severe rash. Regular monitoring may be recommended in long-term therapy.
Patients should be carefully selected for trimethoprim therapy, with attention to folate status, kidney function, and drug interactions.
Trimethoprim has important drug interactions, especially due to its effects on potassium balance and folate metabolism:
Patients should always provide a full list of current medications to their doctor before starting trimethoprim. Even over-the-counter remedies or herbal supplements can sometimes complicate therapy.
Overdose: Symptoms may include nausea, vomiting, dizziness, headache, depression of bone marrow, and metabolic acidosis. In severe cases, folinic acid (leucovorin) may be used as an antidote to counter folate deficiency. Supportive measures, hydration, and monitoring of electrolytes are crucial. Hemodialysis can remove some of the drug.
Accidental double dose: Often not dangerous, though may increase side effect risk (nausea, dizziness). Patients should monitor symptoms and return to normal dosing schedule without taking extra tablets.
Missed dose: Take it as soon as remembered unless close to the next dose, in which case skip the missed dose. Do not double up. Maintaining consistent dosing helps ensure bacterial eradication.
Trimethoprim suspension should be shaken well before each use, and any remaining liquid beyond the expiry date should be discarded. As with all antibiotics, sharing medication is unsafe and should be avoided.
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