Trimethoprim is a synthetic antibacterial agent, closely related to the antimalarial agent pyrimethamine. Chemically Trimethoprim is trimethoxybenzylpyrimidine. It has activity against both gram-positive and gram-negative bacteria. Trimethoprim is commonly used systemically for the treatment and prophylaxis of uncomplicated urinary tract infections, for traveler's diarrhea, and, when combined with either sulfamethoxazole or dapsone, for prophylaxis and treatment of Pneumocystis carinii infections. It was originally made available in combination with sulfamethoxazole in 1973, then was approved by the FDA for use as a single agent in 1980. Antibiotics require constant drug level in body for therapeutic effect. This is achieved by taking the drug at regular intervals of time throughout the day and night as prescribed. Trimethoprim is important to take the drug for the full time period as prescribed. Discontinuation of the drug, may result in ineffective treatment.


Primary Characterstics

Molecular Structure of Trimethoprim
. It is of Synthetic origin and belongs to Pyrimidine. It belongs to Dihydrofolate reductase inhibitor pharmacological group on the basis of mechanism of action and also classified in Antibiotics pharmacological group.The Molecular Weight of Trimethoprim is 290.30.
Its pKa is 7.3.


Oral absorption of Trimethoprim is found to be 96.5% ±1.5. Volume of distribution is found to be 70 - 100 litre and plasma protien binding is 42 - 46 %. Presystemic metabolism is noted to be 0.5% ±0.5 Renal Excretion accounts for 60 - 80 % and plasma half life is 8.6 - 17 hr.



Trimethoprim is contraindicated in conditions like Impaired renal function,Haematological disorders.

Drug Interactions

Trimethoprim is known to interact with other drugs, the details of drug interactions is as follows:

Amantadine (HCl)Renal clearance of amantadine is decrease by trimethoprim result in increase risk of mental confusion due to increase in serum amantadine levels. Minorpatient shoulb be monitor for side effects related to nervous system on coadministration this cobination.
AzathioprineCoadministration increases the risk of bone marrow supression,thrombocytopenia and neutropenia in renal transplant patient.ModerateClose clinical monitoring for hematological adverse effects is recommended during and after therapy.
Colistin (Sulphate & Sulphomethate Na)
Cyclosporin ATrimethoprim decreases the efficacy of cyclosporin.Combined use may increase the serum creatinine level.ModerateClosely monitor the renal function of patient and efficacy and safety of both drugs. An alternative antibiotic may be use.
Ganciclovir (Na)
Metformin (HCl)Trimethoprim reduces the excretion of metformin by competing for renal tubular transport results in increased level of metformin may lead to lactic acidosis.ModerateSlowly reduce the dose of metformin. Closely monitor the blood glucose level. Patient should notify to physician if experience signs of lactic acidosis.
MethotrexateCoadministration enhance thre risk of severe myelosuppression and megoblastic anemia due to synergistic effect involving folate antagonism.MajorCoadministration generally be avoided. If given then close monitoring for hematologic toxicity is recommended.
Para Aminobenzoic AcidShould not be used concurrently except on medical advice.
Phenytoin (Na)
Silver Sulphadiazine
Warfarin (Na)Trimethoprim may decrease the metabolism of Warfarin.ModerateMonitor for increased effects of Warfarin if Trimethoprim is initiated/dose increased, and decreased effects if Trimethoprim is discontinued/dose decreased.

These interactions are sometimes beneficial and sometimes may pose threats to life. Always consult your physician for the change of dose regimen or an alternative drug of choice that may strictly be required.

Interference in Pathology

No data regarding the pathological interferences produced by Trimethoprim is available.

Side Effects

The signs and symptoms that are produced after the acute overdosage of Trimethoprim include Vomiting, Depression, Psychosis, Mania, Facial edema.

Available Brands

Click on the appropriate strength of the dosage form to view its available brands.

Single Ingredient

Susp: 80 mg, 40 mg/5ml,
Tabs: 160 mg, 300 mg,

Multi ingredient

Inj: 160 mg,
Susp: 40 mg/5ml, 80 mg/5ml, 160 mg/5ml,
Tabs: 40 mg, 80 mg, 160 mg, 400 mg,


Trimethoprim's dosage details are as follows:
Dose Single Dose Frequency Route Instructions

Adult Dosage

300 mg300 (300)24 hourlyPOFor 7-Days

Paedriatic Dosage ( 20 Kg. )

4 mg/kg4 (4)12 hourlyOral-

Neonatal Dosage ( 3 Kg. )

4 mg/kg4 (4)12 hourlyOral

High Risk Groups

Drug should not be given to Pregnant Mothers, and Geriatrics.

If prescribing authority justifies the benefits of the drug against the possible damages he/she should reevaluate them and consult the reference material and previous studies.

Warning / Precautions

Trimethoprim should be used with caution in patients with kidney disease, any anemia or any drug allergy. Trimethoprim should be used during pregnancy only if clearly needed. Avoid use during lactation.

Storage Conditions


Store Below 40°C.

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