PREPARATIONS: Tablets: 100 and 200 mg. Solution: 50 mg/5 ml
DRUG INTERACTIONS: Changes in one patient's mental status occurred when a combination of trimethoprim and sulfamethoxazole was given with amantadine (Symmetrel). Blood levels of phenytoin (Dilantin) may be increased by treatment with trimethoprim. This may lead to side effects of phenytoin such as dizziness, and reduced attention. Trimethoprim also may increase blood levels of digoxin (Lanoxin) and warfarin (Coumadin) and lead to serious toxic effects. Anemia, due to a reduction in folic acid, can occur in persons receiving trimethoprim in combination with:
- valproic acid (Depakote, Depakote ER, Depakene, Depacon, Stavzor),
- methotrexate (Rheumatrex, Trexall),
- triamterene, or
The combination of trimethoprim and cyclosporine can increase the risk of kidney damage from cyclosporine. When trimethoprim and dapsone are used together, increased blood concentrations of both drugs can occur, sometimes with side effects that include a toxic condition of the blood called methemoglobinemia. Rifampin can increase the elimination of trimethoprim by the kidneys and may reduce the effectiveness of trimethoprim.
PREGNANCY AND BREASTFEEDING SAFETY: Although there are no human studies that have examined the effects of trimethoprim on the fetus, animal studies have shown adverse effects. Therefore, the physician must weigh the potential risks to the fetus against the potential benefits to the mother when considering trimethoprim therapy for pregnant women.
Trimethoprim is secreted into breast milk in high concentrations. Use of trimethoprim by mothers who are breast-feeding should be avoided.
STORAGE: Tablets should be stored at room temperature, 15 C - 30 C (59 F - 86 F).
DOSING: The usual dose is 100 mg every 12 hours or 200 mg every 24 hours for 10 days. As with all antibiotics, it is important to complete the entire course of trimethoprim even if symptoms improve early during therapy. Persons with kidney diseases may need to receive lower doses since diseased kidneys may not eliminate trimethoprim adequately from the body, and levels of trimethoprim may increase in the body and lead to side effects.
DRUG CLASS AND MECHANISM: Trimethoprim is a synthetic (man-made) antibiotic that interferes with the production of tetrahydrofolic acid, a chemical that is necessary in order for bacteria and human cells to produce proteins. Trimethoprim inhibits production of tetrahydrofolic acid by inhibiting the enzyme responsible for making tetrahydrofolic acid from dihydrofolic acid. Trimethoprim inhibits the bacterial enzyme more than the human enzyme. Therefore, trimethoprim has less effect on the production of tetrahydrofolic acid by humans. Because of the frequent development of resistance to trimethoprim, it is more effective when combined with another antibiotic, sulfamethoxazole (Azo-Gantanol), and is rarely used alone. Trimethoprim was first approved by the FDA in combination with sulfamethoxazole (for example, Bactrim, Septra) in 1973. It was approved as a stand-alone drug in 1980.
Medically reviewed by Eni Williams, PharmD
Reference: FDA Prescribing Information
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