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Bactrim Patient Tips

Written by C. Fookes, BPharm on May 16, 2017

How it works

  • Bactrim is a brand (trade) name for a fixed combination medicine containing two antibiotics - sulfamethoxazole and trimethoprim.
  • Sulfamethoxazole stops bacteria from making dihydrofolic acid and trimethoprim prevents the formation of tetrahydrofolic acid; both significant steps in the formation of nucleic acids and proteins essential to many bacteria.
  • The combination of sulfamethoxazole/trimethoprim is bactericidal (kills bacteria).


  • Bactrim is a combination antibiotic used to treat infections such as those affecting the ear, urinary tract, lungs, or gastrointestinal tract.
  • Active against a wide range of different bacteria.
  • Effective concentrations of both sulfamethoxazole and trimethoprim are reached in the gastrointestinal tract, urinary tract, lungs, mouth, middle ear, and vagina. Both antibiotics also cross the placenta and are excreted in human milk.
  • Bacterial resistance is less likely to develop with Bactrim than if either ingredient (sulfamethoxazole or trimethoprim) is taken alone.


If you are between the ages of 18 and 60, take no other medication or have no other medical conditions, side effects you are more likely to experience include:

  • Nausea, vomiting, abdominal pain, diarrhea, mouth or tongue inflammation, weight loss, flatulence, rash, and itchy skin.
  • May not be suitable for people with kidney or liver disease, folate deficiency (the elderly, chronic alcoholics, and people taking anticonvulsants are at a higher risk of folate deficiency), glucose-6-phosphate deficiency, porphyria, severe allergies, thyroid dysfunction, or bronchial asthma. Elderly people may be more susceptible to the adverse effects of Bactrim.
  • Rarely, severe, sometimes fatal reactions have been reported, such as Stevens-Johnson syndrome (a disorder involving skin and mucous membranes), liver disease, and blood disorders such as thrombocytopenia (low platelets).

Notes: In general, seniors or children, people with certain medical conditions (such as liver or kidney problems, heart disease, diabetes, seizures) or people who take other medications are more at risk of developing a wider range of side effects. For a complete list of all side effects, click here.

Bottom Line

Bactrim is an effective combination antibiotic; however, it may not be suitable for those with kidney or liver disease or folate deficiency. The risk of side effects may be higher in the elderly.


  • Discontinue and seek urgent medical advice if a skin rash develops.
  • Use only as directed by your doctor and do not share with anyone else as misuse can encourage the development of drug-resistant bacteria and reduce the effectiveness of Bactrim.
  • Complete the full course as prescribed (unless instructed not to by your doctor) to reduce the risk of resistant bacteria developing.
  • Should not be used in pregnant or lactating women or in babies less than two months old.
  • Seek urgent medical advice if an unexplained cough, shortness of breath, sore throat, fever, muscle or joint pain, paleness or yellowing of the skin, or diarrhea develop.
  • Like other antibiotics, Bactrim alters the normal flora of the colon and can lead to Clostridium difficile-associated diarrhea, a severe and persistent type of diarrhea. Contact your doctor if diarrhea develops either during treatment or soon after ending treatment.
  • Maintain an adequate fluid intake to avoid the development of crystals in the urine or kidney stones.

Response and Effectiveness

  • Quickly absorbed with peak levels occurring within one to four hours after administration. Antibacterial effects persist for at least 12 hours.


Bactrim (sulfamethoxazole and trimethoprim) [Package Insert]. Revised 01/2014. Caraco Pharma, Inc.

  • Remember, keep this and all other medicines out of the reach of children, never share your medicines with others, and use bactrim only for the indication prescribed.

  • Disclaimer: Every effort has been made to ensure that this information is accurate, up-to-date, and complete, but no guarantee is made to that effect. Drug information contained herein may be time sensitive. This drug information does not endorse drugs, diagnose patients or recommend therapy. It is an informational resource designed as a supplement to, and not a substitute for, the expertise, skill, knowledge and judgment of healthcare practitioners. The absence of a warning for a given drug or drug combination in no way should be construed to indicate that the drug or drug combination is safe, effective or appropriate for any given patient. does not assume any responsibility for any aspect of healthcare administered with the aid of this information. The information contained herein is not intended to cover all possible uses, directions, precautions, warnings, drug interactions, allergic reactions, or adverse effects. If you have questions about the drugs you are taking, check with your doctor, nurse or pharmacist.

Copyright 1996-2017 Revision Date: 2017-05-17 02:14:14