Trimethoprim is an antibiotic that mainly treats bacterial infections affecting the bladder and the kidney. Some health practitioners prescribe it to cure middle ear infections and travelers’ diarrhea.

The effectiveness of Trimethoprim against bacterial infections depends on the type of bacteria and their response to antibiotics in a given geographical area. As such, doctors are advised to look into the antibiogram for guidance on choosing antibiotics for a specific location.

How Does Trimethoprim Work?

Trimethoprim blocks bacterial protein and nucleic acid, thus inhibiting the cellular component-making enzyme Dihydrofolate Reductase. Inhibiting DHFR prevents tetrahydrofolate formation and depletes necessary nucleotides for bacterial nucleic acid synthesis. This, in turn, disrupts the replication and survival of the bacteria pathogen.

DHFR and trimethoprim fight for DHFR’s active site. When trimethoprim binds to the bacteria’s DHFR active site, DHFR cannot convert dihydrofolic acid to tetrahydrofolic acid.

As such, tetrahydrofolate deficiency reduces DNA and RNA synthesis, inhibiting bacterial growth and reproduction. Because of this, trimethoprim kills various gram-negative and gram-positive bacteria.

Trimethoprim Warning and Precautions

  • Hypersensitivity Reactions: Trimethoprim can exacerbate hypersensitivity and allergic reaction symptoms, such as rashes, itching, dizziness, and difficulty breathing.
  • Blood Disorders: In rare cases, Trimethoprim can trigger blood disorders like thrombocytopenia or leukemia. Patients receiving trimethoprim should be closely monitored for signs of blood disorders, such as bleeding and unexplained bruising.
  • Renal Impairment: Trimethoprim is eliminated from the body via the kidneys. If you have renal impairment, your body may struggle to eliminate the drug, leading to drug accumulation and potential toxicity. Consider dose adjustment to avoid adverse effects.
  • Pregnant Women: Trimethoprim can disrupt folate synthesis, leading to folate deficiency in some patients. This might lead to pregnancy-related complications.
  • Antibiotic resistance: Prolonged use of antibiotics may lead to the development of antibiotic-resistant bacteria. Observe proper drug transcription and complete the dose even when symptoms disappear.
  • Geriatric and Pediatric Considerations: Trimethoprim metabolism, renal function, and adverse effects may differ in geriatric and pediatric patients. Practice caution when administering Trimethoprim to these individuals.

Trimethoprim Side Effects

Trimethoprim has adverse effects like any drug. Any unusual or severe side effects must be reported to the doctor promptly. Patients should follow dosage and treatment duration directions to reduce side effects. Common trimethoprim side effects include:

Common Side Effects

  • Gastrointestinal Side Effects: You may experience signs such as nausea, vomiting, diarrhea, and abdominal pain.
  • Skin reactions: Trimethoprim hypersensitivity may cause urticaria, itching, and rash.
  • Allergic Reactions: The drug can cause rash, irritation, or anaphylaxis in vulnerable people. Allergic people should be monitored closely.
  • Photosensitivity: You may notice sunburn-like responses or skin discoloration. Patients should utilize sunscreen and protective clothes when outside.

Severe Side Effects

  • Thrombocytopenia or Leukopenia: The drug may cause a decrease in WBC and platelets (thrombocytopenia or leukopenia), which can cause symptoms like fever or bruises.
  • Central Nervous System Effects: Trimethoprim may cause headaches, dizziness, and insomnia. While these may go away independently, seek assistance if persistent or severe.

Trimethoprim Drug Interactions

Drug interactions with trimethoprim may reduce efficacy or cause adverse effects. Meaningful medication interactions include:

Warfarin and Other Anticoagulants

Trimethoprim increases the risk of bleeding caused by Warfarin. When used together with anticoagulants, trimethoprim needs INR monitoring and dosage modifications to mitigate these risks.

ACE inhibitors or ARBs

When used together with ACE inhibitors or ARBs, trimethoprim can raise serum potassium levels, increasing the risk of hyperkalemia.


Trimethoprim reduces renal tubular methotrexate release, increasing methotrexate concentration in the blood and potentially increasing its toxicity.


Trimethoprim increases digoxin levels by blocking renal clearance. This can result in digoxin toxicity and cause nausea, vomiting, dizziness, and arrhythmia.

Potassium-Raising Drugs

Just as with ACE inhibitors, combining trimethoprim with potassium-sparing diuretics (spironolactone, amiloride) and other potassium-raising drugs might cause hyperkalemia.


Trimethoprim inhibits cyclosporine (found in Restasis) metabolism and may, therefore, elevate serum levels and cause toxicity. Combining these drugs may require cyclosporine monitoring and dosage adjustments.

Trimethoprim Dosages and Strengths

Trimethoprim is available in tablet form and oral suspension(for pediatric use), which have different strengths. As directed by a physician, the dosage may vary depending on patient variables and clinical circumstances.

Tablets100 mgAdults: 100 mg every 12 hours
Pediatric: Dosage based on weight and age

(2 mg/kg/day divided every 12 hours) up to a Max of 5 mg/kg/day

200 mgAdults: 200 mg every 12 hours
Pediatric: Dosage based on weight and age

(4 mg/kg/day divided every 12 hours) up to a Max of 10 mg/kg/day

Suspension40 mg/5 mLPediatric: 8 mg/kg/day divided every 12 hours up to a Max of 12 mg/kg/day

FAQs of Trimethoprim 

What should I do in case I skip a Trimethoprim dose?

If you forget to take a dose of Trimethoprim, it is recommended that you take the missed dose as soon as possible. However, if the next dose is close, you can ignore the missed dose and continue with the scheduled dose.

Can I take Trimethoprim while pregnant?

You can use Trimethoprim when pregnant. However, you should consult your healthcare provider on the correct dosage and strength for expectant mothers.

Is Trimethoprim a strong antibiotic?

Yes! Trimethoprim is a robust antibacterial agent commonly used to treat kidney and bladder bacterial infections.

Can trimethoprim affect my menstrual cycle?

Yes! Trimethoprim is an antibiotic used to treat UTIs affecting the uterus. As such, it can sometimes trigger hormonal imbalances that might interfere with your menstrual cycle.

Can I take Trimethoprim alongside other drugs?

It’s safe to use trimethoprim with other drugs. However, check for interactions, as it can react with certain medications, resulting in unexpected side effects.

What is the cost of Trimethoprim in America?

The cost of Trimethoprim may vary depending on the dosage and strength prescribed and the pharmacy you visit. On average, it costs $68 for a supply of 100 tablets of 100 mg. Note that these prices only apply to cash-paying patients, not insurance programs, discounts, or coupons.

Are there alternatives to Trimethoprim?

Trimethoprim can be replaced with other alternative antibiotics such as Ciprofloxacin, Levofloxacin, Doxycycline, and Amoxicillin.

IMPORTANT NOTE: The information provided here is for educational purposes only and is not intended to serve as medical advice, diagnosis, or treatment recommendations. It should not be taken as an endorsement of any specific medication or treatment. Individual health conditions and responses to treatment can vary greatly; therefore, this information should not be seen as a guarantee of safety, suitability, or effectiveness for any particular individual. Always consult with a healthcare professional for personalized medical advice and before making any decisions regarding your health or treatment plans.

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