Relieve UTIs with generic Bactrim DS antibiotics on prescription

Trimethoprim (generic Bactrim) is frequently used to treat UTIs, as it kills the bacteria quickly and makes you feel better within a day or two.

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Reviewed by Patrick Moser, FNP-BC

Information last reviewed 08/20/19


What is Trimethoprim?

Trimethoprim is a type of antibiotic. Its main purpose is to treat urinary tract infections (UTIs) but might also be given for chest infections or acne. 

How does Trimethoprim work?

Trimethoprim works by stopping the bacteria that causes the infection from being able to multiply and survive, resulting in the disappearance of the bacterial infection. 

How long does it take for Trimethoprim to work?

Your symptoms will usually start to ease off after a few days of starting Trimethoprim. Always complete the full course of the treatment to ensure the infection is properly treated. 

What are the symptoms of a UTI?

The symptoms of a UTI commonly include: 

  • An urgent need to urinate 

  • Frequent urination 

  • Burning feeling or pain while urinating 

  • Urine that's cloudy, smelly or bloody

  • Lower abdominal pain

  • Tiredness 

In elderly people, they may experience behavioral changes such as confusion and agitation. 

Other uses of Trimethoprim

The antibiotic is most commonly used for bacterial infections like UTIs. It can also be given to prevent UTIs from recurring in people who get them frequently. Aside from this, the medication can also be useful in the treatment of chest infections and acne. 


Active ingredients

The active ingredient in Trimethoprim is trimethoprim.

Inactive ingredients

The inactive ingredients in Trimethoprim are colloidal silicon dioxide, lactose, macrogol, magnesium stearate, povidone, sodium starch glycollate, stearic acid, and microcrystalline cellulose (E460). (Please note: inactive ingredients may vary across different manufacturers).

Which ingredients can cause an allergic reaction?

This medication does not contain any active or inactive ingredients associated with causing an allergic reaction in users. However, if you are using the medication for the first time, please lookout for signs of anaphylaxis (facial swelling, difficulty breathing, dizziness, and rash) or a severe skin reaction (burning eyes, sore throat, and fever). If you notice any of these symptoms, please seek immediate medical attention.


What should I do if I take too many Trimethoprim tablets?

If you take too many doses of Trimethoprim, seek immediate medical attention. 

Do I need to finish the course of Trimethoprim even if my symptoms have resolved?

You need to take all the doses of Trimethoprim that you have been given otherwise, the infection may not be fully cleared. This leaves you at risk of a repeat UTI. 

Trimethoprim dosage

The dosage may vary depending on your symptoms and the nature of your infection. Always take the dose prescribed by your doctor.  

How to take Trimethoprim

Trimethoprim should be taken at the same time each day, and usually twice daily. Ensure you spread out the doses, taking one pill in the morning and the other in the evening. Each one should be swallowed whole with water. 

What should I do if I miss a dose of Trimethoprim?

If you forget a dose of Trimethoprim, take it as soon as you can. If it's nearly time for your second dose then leave the missed one. You should never double up a dose. 

When can I stop taking Trimethoprim?

Take Trimethoprim for as long as your doctor tells you to. This will vary from one individual to another. 

Side Effects

Trimethoprim side effects

Common side effects include: 

  • Heightened levels of potassium in the blood

  • Nausea

  • Vomiting 

  • Diarrhea 

  • Headache

  • Rash

  • Urticaria 

  • Anemia 

  • Sore throat

  • Mouth ulcers 

  • Thrush 

Rare side effects may include: 

  • Constipation 

  • Severe diarrhoea 

  • Coughing 

  • Altered behaviour 

  • Aggression 

  • Aseptic meningitis (signs of this include stiff neck, headache, fever, feeling generally unwell, tiredness and sensitivity to light)

  • Kidney problems 

  • Dizziness 

  • Tiredness 

  • Convulsions 

  • Pins and needles 

  • Shakiness or tremors 

  • Vertigo and tinnitus 

  • Pain or redness in the eyes

  • Photosensitivity 

  • Low blood sugar 

  • Low sodium 

  • Anorexia 

  • Depression 

  • Anxiety 

  • Insomnia 

  • Confusion 

  • Jaundice 

  • Aching muscles and joints 

  • Shortness of breath

  • Wheezing

  • Nosebleeds 

  • Lupus erythematosus 

  • Skin discoloration 

  • Fever 

Severe side effects which need immediate medication attention include: 

  • Swollen face, lips, tongue or throat 

  • Difficulty breathing 

  • Chest pain 

  • Fainting or shock 

  • Peeling or blistering of the skin 

  • Lesions or skin eruptions 

  • Pancreatitis


Trimethoprim is not suitable for individuals who: 

  • Are allergic to any of the ingredients in the medicine 

  • Are pregnant 

  • Have a blood disorder 

  • Have severe kidney or liver disease 

Tell your doctor if: 

  • You are deficient in folic acid 

  • You have kidney disease 

  • You are having dialysis treatment 

Drug interactions

The following medications might interact with Trimethoprim: 

  • Other antibiotics 

  • Anticoagulants 

  • Digoxin 

  • Phenytoin 

  • Pyrimethamine 

  • Dapsone 

  • Immunosuppressant drugs 

  • Bone marrow depressants 

  • Spironolactone 

  • Repaglinide 

  • Diuretics 

  • Procainamide 

Treatment Options

Trimethoprim vs Nitrofurantoin

Both Trimethoprim and Nitrofurantoin are antibiotics used to treat UTIs. Nitrofurantoin is used solely for this function as the medicine is filtered straight to the urine. If you have low levels of folic acid in your blood or you are pregnant, then Nitrofurantoin is more suitable. Trimethoprim is likely to be prescribed if you have diabetes or a vitamin B deficiency. 

Alternatives to Trimethoprim

There are several types of antibiotics that treat urinary tract infections. Quinolone types like Nitrofurantoin, Ciprofloxacin, Norfloxacin and Ofloxacin are the first choice of medicines to treat UTIs. Trimethoprim may be prescribed if these types of antibiotics are not suitable, or prove ineffective. 


Disclaimer: This is not medical advice. You and your physician will determine if and how you should take any medication prescribed to you following a medical consultation.

  1. (NHS Choices) <> accessed July 28, 2019 
  2. Gleckman R, Blagg N and Joubert DW, “Trimethoprim: Mechanisms of Action, Antimicrobial Activity, Bacterial Resistance, Pharmacokinetics, Adverse Reactions, and Therapeutic Indications” (1981) 1 Pharmacotherapy 14 
  3. Lee M and others, “Urinary Tract Infections in Pregnancy” (2008) 54 Canadian Family Physician 853 
  4. “Trimethoprim: MedlinePlus Drug Information” (MedlinePlus <> accessed July 28, 2019

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