This is not a complete list of side effects and others may occur.  

More common:

• diarrhea
• nausea, vomiting
• stomach upset
• loss of appetite
• changes in taste
• headache 

If any of these effects persist or worsen, notify your doctor or pharmacist promptly.

Tell your doctor right away if you have any of these serious side effects:

• swollen tongue
• signs of infection (such as fever, persistent sore throat)
• easy bruising/bleeding
• unusual paleness
• mental/mood changes
• tiredness with fast/pounding/irregular heartbeat
• dark urine
• stomach pain,
• jaundice

Trimethoprim prevents the formation of tetrahydrofolate, a compound that bacteria must be able to make in order to survive.

The effectiveness of trimethoprim depends on factors such as dose, concentration in the blood as well as other body fluids and tissue, and susceptibility of the organism. It will only be effective against certain types of bacteria sensitive to its antibiotic action. Infections caused by bacteria that are not sensitive to trimethoprim will not show improvement after taking this medication.

Do not use trimethoprim:

• If you have an allergy to trimethoprim or any other part of this drug.

Use trimethoprim with caution:

• If you have anemia caused by a lack of folic acid.

This drug may affect certain lab tests. Be sure your doctor knows you take this drug.

You may get sunburned more easily. Avoid sun, sunlamps, and tanning beds. Use sunscreen and wear clothing and eyewear that protects you from the sun.

Drug interactions:

• Dofetilide side effects, including severe irregular heartbeat, may be increased by trimethoprim
• Digoxin, phenytoin, methotrexate -  risk of their side effects may be increased by trimethoprim

Use in pregnancy: Trimethoprim crosses the placenta.  Use is not recommended in pregnancy unless benefit outweighs risk.  Consult your doctor or pharmacist before use.

Use in breastfeeding: Trimethoprim is passed into breast milk in small amounts.  Consult your doctor or pharmacist before use.