Trimethoprim Side Effects – Your Questions Answered

Trimethoprim is a commonly prescribed antibiotic in the UK. It treats various bacterial infections, most commonly urinary tract infections (UTIs) such as cystitis. This post will provide you with essential information about trimethoprim, including its uses, dosage, trimethoprim side effects, and important precautions.

What is Trimethoprim?

Trimethoprim is an antibacterial medicine that prevents bacteria from multiplying and growing, helping the body’s immune system fight off infection.

Trimethoprim is commonly prescribed for:

• Acute urinary tract infections (UTIs): This includes infections of the bladder (cystitis), kidneys (pyelonephritis), and urethra.

• Recurrent UTIs: For individuals who experience UTIs frequently, trimethoprim may be used to prevent future infections.

• Other infections: While less common, trimethoprim can also treat certain skin infections, respiratory infections, ear infections, and traveller’s diarrhoea.

How to Take Trimethoprim

Potential trimethoprim side effects can be reduced or completely avoided by carefully following your doctor’s instructions and the information that comes with the medication. Here are some general guidelines:

• Dosage: The usual dose for adults with severe or sudden infections is 200mg twice daily. However, your doctor will determine the appropriate dosage based on your individual needs and the type of infection you have.

• Administration: Trimethoprim tablets should be swallowed whole with a glass of water. Do not chew the tablets.

• Timing: Try to take your doses simultaneously each day to maintain a consistent level of medicine in your system.

• Missed dose: If you forget to take a dose, take it as soon as you remember. However, if it’s almost time for your next dose, skip the missed dose and continue with your regular schedule. Do not take a double dose to make up for a missed one.   

• Duration of treatment: Complete the full course of treatment prescribed by your doctor, even if you start to feel better. Stopping treatment early may lead to the infection returning or becoming more difficult to treat (trimethoprim resistance).

Trimethoprim Side Effects and Precautions

Trimethoprim side effects are rare because it is considered to be generally safe, but like all medicines, it can cause side effects in some people.

Common side effects include:

• Nausea
• Vomiting
• Diarrhea
• Rash
• Itching
• Headache

There can be more serious trimethoprim side effects that are particularly rare but should never be ignored, including:

• Severe allergic reactions (anaphylaxis): Symptoms include difficulty breathing, swelling of the face, lips, tongue, or throat, chest pain, shock, fainting, or collapse. Seek immediate medical attention if you experience any of these.

• Blood disorders: Trimethoprim can rarely cause a decrease in certain types of blood cells, which may lead to anaemia, fatigue, or increased risk of infections.

• Liver problems: Symptoms might include jaundice (yellowing of the skin or eyes), dark urine, or abdominal pain.

• Kidney problems: Although trimethoprim is used to treat kidney infections, it can sometimes cause kidney problems, especially in people with pre-existing kidney disease.

Before starting trimethoprim, it is crucial to discuss your medical history with your doctor. Pregnant or breastfeeding women should exercise caution, as trimethoprim may pose risks to the developing baby or pass into breast milk. If you have any kidney problems or are undergoing dialysis, your doctor must be made aware of this, as trimethoprim can affect kidney function.

Additionally, inform your doctor if you have a folate deficiency (vitamin B9), as this medication can interfere with folate metabolism. Individuals at risk of high potassium levels (hyperkalemia), such as the elderly or those taking higher doses, should also be closely monitored while on trimethoprim.

It is essential to disclose all other medications you are taking, including those not prescribed by your doctor. Trimethoprim can interact with certain medications, particularly warfarin, methotrexate, cyclosporine, and other antibiotics. These interactions may increase the risk of side effects or reduce the effectiveness of your medications. Finally, if you have porphyria, a rare blood disorder, trimethoprim medication may not be suitable for you.

As a general precaution, it is advisable to avoid or limit alcohol consumption while taking any medication, including trimethoprim. Alcohol can potentially worsen certain side effects or interact with the medication in unexpected ways.

Trimethoprim side effects, Long-Term

In most cases, trimethoprim is prescribed for short-term use. However, long-term use may increase the risk of certain side effects, including:

• Bone marrow suppression: This can lead to a decrease in the production of blood cells.
• Folate deficiency: Trimethoprim can interfere with the body’s use of folate.
• Peripheral neuropathy: This condition affects the nerves, causing numbness, tingling, or pain in the hands and feet.

Trimethoprim Allergy and resistance

If you have a known allergy to trimethoprim, you should not take this medication. Symptoms of an allergic reaction can range from mild (rash, itching) to severe (anaphylaxis).

Overuse or inappropriate use of antibiotics can lead to antibiotic resistance, where bacteria become resistant to the medication’s effects. This makes infections more difficult to treat. It is crucial to only take trimethoprim when prescribed by your own doctor or one of our medical prescribers at Access Doctor and to complete the full course of treatment.

Alternative Treatments for UTIs

In some cases, alternative treatments for UTIs may be considered after your medical consultation with an Access Doctor. Our medical team may recommend another treatment, such as another antibiotic, such as nitrofurantoin or cephalexin. Your Access Doctor will suggest the most appropriate antibiotic based on your circumstances.

Certain home remedies, such as drinking plenty of fluids, cranberry juice, and using a hot water bottle to ease pain, may help relieve UTI symptoms for those with mild symptoms or who are sensitive to this medication and want to avoid trimethoprim side effects. However, these may prove ineffective and should not replace medical treatment.

Reporting Side Effects

If you experience any side effects while taking trimethoprim, it is important to report them to your doctor or pharmacist. You can also report side effects directly via the Yellow Card Scheme at www.mhra.gov.uk/yellowcard.