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Your child needs to take the medicine called trimethoprim (say: trye-METH-oh-prim). This information sheet explains what trimethoprim does, how to give it, and what side effects or problems your child may have when he or she takes this medicine.

What is trimethoprim?

Trimethoprim is a medicine called an antibiotic. Antibiotics are used to treat or prevent certain types of infections caused by the germs called bacteria. Trimethoprim comes as a tablet or liquid.

Before giving trimethoprim to your child...

  • Tell your doctor if your child has ever had any unusual or allergic reaction to trimethoprim.

Talk with your doctor or pharmacist if your child has any of the following conditions. Precautions may need to be taken with this medicine if your child has:

  • liver or kidney disease
  • anemia (low red blood cell counts)

How should you give your child trimethoprim?

Follow these instructions when giving your child trimethoprim:

  • Give your child this medicine for as long as your doctor or pharmacist tells you to, even if your child seems better before that time.
  • Give trimethoprim at the same time(s) every day. Pick times that are easy for you so that you do not miss doses.
  • Give this medicine on an empty stomach (one hour before meals or two hours after meals) with a full glass of liquid. If it upsets your child's stomach, it may be taken with food.
  • If your child is taking the liquid form of trimethoprim, measure it with the special spoon or syringe that the pharmacist gave you. Shake the bottle well before you measure the dose.
  • If your child is taking the tablet and can not swallow it, it can be crushed and mixed with a small amount of food or liquid.

What should you do if your child misses a dose of trimethoprim?

If your child misses a dose of trimethoprim:

  • Give your child the missed dose as soon as you remember.
  • If it is almost time for the next dose, skip the missed dose. Give your child the next dose at the regular time.
  • Give your child only one dose at a time.

How long does trimethoprim take to work?

Your child may start feeling better several days after starting this medicine.

What are the possible side effects of trimethoprim?

Your child may have some side effects while he or she takes trimethoprim. Usually, your child will not need to see a doctor about them. These side effects may go away as your child's body gets used to trimethoprim:

  • loss of appetite
  • stomach cramps and pain
  • watery bowel movements (diarrhea)
  • upset stomach, throwing up
  • headache

Most of the following side effects are not common, but they may be a sign of a more serious problem. Call you child's doctor right away or take your child to the Emergency Department if your child has any of these symptoms:

  • unusual bleeding or bruising
  • slow healing of cuts
  • sore throat and fever
  • severe headache, neck stiffness
  • aching joints and muscles
  • red, blistering, or peeling skin
  • rash or itchy skin
  • bluish fingernails, lips, or skin
  • trouble breathing
  • unusual tiredness or weakness
  • confusion
  • yellow skin or eyes
  • dark urine

What safety measures should you take when your child is using trimethoprim?

  • Trimethoprim may make your child's skin more likely to sunburn. Cover your child's head with a hat when outside. He or she should also use a sunscreen. Ask your pharmacist to help you pick the right sunscreen.
  • Trimethoprim may cause low blood counts. Your child may need blood tests if your child is taking this medicine for a long time. If your child's blood counts are low, your doctor, nurse, or pharmacist will tell you how your child can try to avoid infections and bleeding.
  • Trimethoprim may not work well with other medicines. Check with your child's doctor or pharmacist before giving your child any other medicines (prescription, non-prescription, herbal, or natural products).
  • Tell the doctor or dentist that your child is taking trimethoprim before he or she has any kind of operation, even on the teeth or an emergency treatment.

What other important information should you know about trimethoprim?

Trimethoprim liquid must be made by a pharmacy. Make sure your pharmacy is able to make it for your child.

Make sure you always have enough trimethoprim to last through weekends, holidays, and vacations. Call your pharmacy at least 2 days before your child runs out of medicine to order refills.

Keep trimethoprim in its original bottle and store it at room temperature in a cool, dry place away from sunlight. Do not store it in the bathroom or kitchen.

Keep a list of all medications your child is on and show the list to the doctor or pharmacist.

Do not share your child's medicine with others. Do not give anyone else's medicine to your child.

Do not keep any medicines that are out of date. Check with your pharmacist about the best way to throw away outdated or leftover medicines.

Keep trimethoprim out of your child's sight and reach and locked up in a safe place. If your child takes too much trimethoprim, call the Ontario Poison Centre at one of these numbers. These calls are free.

  • Call 416-813-5900 if you live in Toronto.
  • Call 1-800-268-9017 if you live somewhere else in Ontario.
  • If you live outside of Ontario, call your local Poison Information Centre.

Disclaimer: The information in this Family Med-aid is accurate at the time of printing. It provides a summary of information about trimethoprim and does not contain all possible information about this medicine. Not all side effects are listed. If you have any questions or want more information about trimethoprim, speak to your health care provider.

Jennifer Drynan-Arsenault, BSc, RPh, ACPR