What is Primsol (trimethoprim) and how does it work?

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Primsol 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. Primsol inhibits the bacterial enzyme more than the human enzyme. Therefore, Primsol has less effect on the production of tetrahydrofolic acid by humans. Because of the frequent development of resistance to Primsol, it is more effective when combined with another antibiotic, sulfamethoxazole (Azo-Gantanol), and is rarely used alone.

What brand names are available for trimethoprim?

  • Primsol is the brand name for trimethoprim  available in the US.
  • Trimpex and Proloprim are brand names that have been discontinued in the US.

Is Primsol (trimethoprim) available as a generic drug?


Do I need a prescription for Primsol (trimethoprim)?


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What are the uses for Primsol (trimethoprim)?

  • Primsol is used for the treatment of uncomplicated urinary tract infections (UTI) due to susceptible bacteria such as Escherichia coli, Proteus mirabilis, Klebsiella pneumoniae, Enterobacter species, and coagulase-negative Staphylococcus species, including S. saprophyticus.

What are the side effects of Primsol (trimethoprim)?

The most common side effects associated with Primsol involve the gastrointestinal (GI) tract, such as:

Other important side effects are:

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Rarely, the skin 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.

Serious side effects include:

Primsol may cause anemia due to a deficiency of folic acid. The anemia usually is mild and resolves when the Primsol is stopped. Patients who are folate-deficient, such as malnourished, alcoholic, geriatric, or pregnant females, may be at greater risk for developing anemia with Primsol. 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.

What is the dosage for Primsol (trimethoprim)?

  • 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 Primsol even if symptoms improve early during therapy.
  • Persons with kidney diseases may need to receive lower doses since diseased kidneys may not eliminate Primsol adequately from the body, and levels of Primsol may increase in the body and lead to side effects.

Which drugs or supplements interact with Primsol (trimethoprim)?

  • 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, Jantoven) and lead to serious toxic effects.
  • Anemia, due to a reduction in folic acid, can occur in persons receiving trimethoprim in combination with:
  • 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.

Is Primsol (trimethoprim) safe to use during pregnancy or while breastfeeding?

  • 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.

What else should I know about Primsol (trimethoprim)?

What preparations of Primsol (trimethoprim) are available?
  • Tablets: 100 and 200 mg.
  • Solution: 50 mg/5 ml
How should I keep Primsol (trimethoprim) stored?
  • Tablets should be stored at room temperature, 15 C – 30 C (59 F – 86 F).
When was Primsol (trimethoprim) approved by the FDA?
  • Trimethoprim was first approved by the FDA in combination with sulfamethoxazole (for example, Bactrim, Septra) in 1973.
  • Trimethorpim was approved as a stand-alone drug in 1980.

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Medically Reviewed on 3/15/2019


Medically reviewed by John P. Cunha, DO, FACOEP; Board Certified Emergency Medicine


FDA Prescribing Information