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What you need to know about the drug Trimethoprim

MBBS
Medically Reviewed by MBBS ✓ Medically Reviewed
📅 Updated: Jul 14, 2026 • ⏱ 7 min read • Editorial policy
What you need to know about the drug Trimethoprim
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Sometimes, your doctor will prescribe a liquid medicine to treat a bacterial infection for you or your baby, right? One such medicine is Trimethoprim. It is an antibiotic, meaning it kills bacteria in the body. So today, let's talk about this medicine and what you need to be careful about when using it.

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What exactly is this medicine used for?

Simply put, Trimethoprim is an antibiotic that treats certain types of bacterial infections .

Remember one very important thing. This medicine does not work for viral infections . That means it will not cure things like the common cold or flu. Your doctor will only prescribe it if they are certain that you have a bacterial infection.

This medication may come under different brand names, such as Primsol or TRIMPEX, but they all contain the same active ingredient, trimethoprim.

Things you need to tell your doctor before taking medicine

Before you use this medicine, it is important to tell your doctor the following information. This is very important for your safety.

Situation to be reported Why is that important?
If you have anemia This medicine may affect some anemia conditions.
If you have kidney or liver disease Since medication is eliminated from the body through the kidneys and liver, if there is a problem with these organs, the dosage of the medication will have to be changed.
If there is a nutritional deficiency or malnutritionBecause this medication affects the vitamin folic acid, it can affect people who already have nutritional problems.
If you have an allergic reaction to trimethoprim or other medicines, foods, or dyes. This information is essential to prevent a serious allergy.
If you are pregnant, trying to get pregnant, or breastfeeding Since this medication can affect the baby in the womb and can pass to the baby through breast milk, it is essential to discuss this with your doctor.

How do you use this medicine?

  • This medicine is an oral liquid. Take it exactly as prescribed by your doctor, at the time.
  • Use only the medicine cup, oral syringe, or special spoon provided by your doctor to measure the correct dose . Do not use household teaspoons or tablespoons. They are not accurate. Therefore, the amount of medicine given may be less or more.
  • Use the medicine for the full number of days prescribed by your doctor. Do not stop taking it just because you feel better. Doing so may cause the infection to come back.
  • If you are giving this medicine to your child, especially if he or she is under 6 months old, follow your doctor's instructions exactly.

What if you take too much medicine? (Overdose)

If you think you have taken more medicine than you think, immediately call the National Poisons Information Centre at the Colombo National Hospital . Or go to the Emergency Treatment Unit (ETU) of the nearest hospital.

Important: This medicine has been prescribed for you only. Do not give it to anyone else who has the same symptoms as you.

What to do if you miss a dose of medicine?

Take the missed dose as soon as you remember. However, if it is almost time for your next dose, skip the missed dose and take your next dose at the scheduled time. Never take two doses at once to make up for a missed dose.

Other medications that may interact with this medicine (Interactions)

This is a very important part. You should tell your doctor about all the medications you are taking. This includes over-the-counter medications, vitamins, herbal remedies, and anything else you are taking.

Never take this with the following medications:

  • dofetilide (a medicine for heart rhythm problems)

This medicine may interact with the following medications. Therefore, tell your doctor if you are taking these:

  • ACE inhibitors: These are a type of medication used to treat high blood pressure (e.g. enalapril, lisinopril).
  • dapsone
  • digoxin (for heart disease)
  • fluorouracil (for cancer)
  • MAOIs: Some medications used to treat mental illness.
  • Medicines that prevent blood clotting, for example warfarin .
  • phenytoin (for epilepsy)
  • procainamide
  • pyrimethamine
  • Some diuretics
  • trimetrexate

This list is not complete. Therefore, it is essential to inform your doctor about all medications you are taking.

Things to keep in mind when using the medicine

  • Attend clinics as prescribed by the doctor.
  • If your symptoms do not improve or worsen, tell your doctor immediately.
  • Be careful with the sun: Your skin may become very sensitive to the sun (photosensitivity) while taking this medicine. So avoid going out in the sun as much as possible. If you do go out, wear sun-protective clothing, good sunglasses, and use a suitable sunscreen after consulting your doctor. Do not use sun lamps or tanning beds.

Side effects that may occur due to medication

Like any medicine, trimethoprim can cause side effects. Some are serious, some are common.

Side effects that should be reported to your doctor immediately
  • Allergy symptoms: skin rash, itching, swelling of the face, lips, or tongue.
  • Difficulty breathing.
  • Chest pain.
  • Fast or irregular heartbeat.
  • Fever, chills, sore throat.
  • Redness, blistering, or peeling of the skin, including inside the mouth.
  • Unusual bleeding or bruising of the skin.
  • Feeling unusually tired or weak.
Side effects that should be reported to the doctor if they persist or are bothersome
  • Body pain.
  • Change in taste in the mouth.
  • Diarrhea.
  • Headache.
  • Nausea or vomiting.
  • Stomach pain.

This list does not include all possible side effects. If you notice anything unusual, talk to your doctor immediately.

How to keep medicine safe?

  • Keep it out of the reach of children.
  • Store at room temperature (between 15 and 25 degrees Celsius).
  • Protect from light and moisture. That means don't keep it in damp places like the bathroom.
  • Keep the cap of the medicine bottle tightly closed.
  • Throw away expired medicine.

💡 Take-Home Message

  • Trimethoprim is an antibiotic used to treat bacterial infections. It does not work for the common cold.
  • Use the medicine for the full number of days prescribed by the doctor. Do not stop taking it even if your symptoms improve.
  • Be sure to tell your doctor about all other medications, vitamins, and medical conditions you are taking.
  • Protect your skin from the sun while taking this medication, as it may make your skin sensitive to the sun.
  • If serious side effects (difficulty breathing, skin rashes, swelling) occur, seek medical advice immediately.
  • In case of any problems, talk to your doctor.
❓ FAQ

FAQ

Evidence-based clinical answers to common patient questions

What to do if you miss a dose of medicine?

Take the missed dose as soon as you remember. However, if it is almost time for your next dose, skip the missed dose and take your next dose at the scheduled time. Never take two doses at once to make up for a missed dose.

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⚠️ Important: Medical articles and information on Nirogi Lanka are strictly for educational and awareness purposes and do not substitute professional medical advice, diagnosis, or treatment. Always consult a qualified physician for any medical concerns.