Pharmacy Editor: Jay W. Marks, MD
GENERIC NAME: trimethoprim
BRAND NAMES: Trimpex; Proloprim, Primsol
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 corresponding human
enzyme. Therefore, trimethoprim has less effect on the production of
tetrahydrofolic acid by humans. Trimethoprim is effective against a wide variety
of bacteria. Trimethoprim was first approved by the FDA in combination with
sulfamethoxazole (e.g., Bactrim, Septra) in 1973. It was approved as a
stand-alone drug in 1980.
PRESCRIPTION: Yes
GENERIC AVAILABLE: Yes
PREPARATIONS: Tablets: 100, and 200 mg. Solution: 50 mg/5 ml
STORAGE: Tablets should be kept at room temperature, 15-30°C
(59-86°F).
PRESCRIBED FOR: Trimethoprim is used for the treatment of urinary
tract infections, traveler's diarrhea, respiratory and middle ear infections,
and, when combined with either sulfamethoxazole or dapsone, for prevention and
treatment of Pneumocystis infections.
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 in therapy. Persons
with kidney diseases may need to receive lower doses. (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 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 associated with 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 divalproex, valproic acid, methotrexate,
pyrimethamine, triamterene, or trimetrexate.
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 called methemoglobinemia.
Rifampin can increase the elimination of trimethoprim by the kidneys and may
reduce the effectiveness of trimethoprim.
PREGNANCY: 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.
NURSING MOTHERS: Trimethoprim is distributed into breast milk in high
concentrations. Use of trimethoprim by mothers who are breast-feeding should be
avoided.
SIDE EFFECTS: The most common side effects associated with
trimethoprim are gastrointestinal. They include loss of appetite, nausea,
vomiting, diarrhea, abdominal pain, abnormal taste, and swelling of the tongue.
Taking trimethoprim with food may reduce some of these side effects.
Trimethoprim may cause anemia due to a deficiency of folic acid. The anemia
usually is mild and resolves when the trimethoprim is stopped. Patients who are
folate-deficient, such as malnourished, alcoholic, geriatric, or pregnant
patients, may be at greater risk for developing anemia with trimethoprim.
Warning signs of anemia include white or bluish fingernails and unusual
tiredness and weakness. Prolonged therapy can result in low platelet counts, low
white blood cell counts, and other toxic effects on the blood cells.
The most common side effects related to the skin are rash and itching.
Rarely, the rash can progress to peeling or blistering. Some patients have
photosensitivity reactions, that is, they develop skin rashes on parts of their
body that are exposed to the sun.
Last Editorial Review: 4/18/2006
Report Problems to the Food and Drug Administration
You are encouraged to report negative side effects of prescription drugs to the FDA. Visit the FDA MedWatch website or call 1-800-FDA-1088.
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