Many medications can cause side effects. A side effect is an unwanted response to a medication when it is taken in normal doses. Side effects can be mild or severe, temporary or permanent.

The side effects listed below are not experienced by everyone who takes Trimethoprim. If you are concerned about side effects, discuss the risks and benefits of Trimethoprim with your doctor.

The following side effects have been reported by at least 1% of people taking Trimethoprim. Many of these side effects can be managed, and some may go away on their own over time.

Contact your doctor if you experience these side effects and they are severe or bothersome. Your pharmacist may be able to advise you on managing side effects.

• increased sensitivity to the sun
• itching
• mouth sores
• muscle weakness
• nausea
• skin rash
• stomach cramps or pain
• vomiting

Although most of the side effects listed below don't happen very often, they could lead to serious problems if you do not seek medical attention.

Check with your doctor as soon as possible if any of the following side effects occur:

• general feeling of discomfort or illness
• headache
• redness, swelling, or soreness of tongue
• signs of anemia (low red blood cells; e.g., pale skin, unusual tiredness or weakness)
• signs of bleeding (e.g., bloody nose, blood in urine, coughing blood, cuts that don't stop bleeding)
• signs of infection (symptoms may include Fever or chills, severe Diarrhea, shortness of breath, prolonged dizziness, headache, stiff neck, weight loss, or listlessness)
• signs of liver problems (e.g., nausea, vomiting, diarrhea, loss of appetite, weight loss, yellowing of the skin or whites of the eyes, dark urine, pale stools)
• symptoms of low sodium in the blood (e.g., tiredness, weakness, confusion, achy, stiff, or uncoordinated muscles)
• symptoms of too much potassium in the body (e.g., muscle fatigue, weakness, difficulty moving, abnormal heart rhythms, nausea)
• unusual tiredness or weakness

Stop taking the medication and seek immediate medical attention if any of the following occur:

• signs of a serious allergic reaction (e.g., difficulty breathing, Hives, or swelling of the face and throat)
• unusual skin reactions (e.g., pinpoint-sized red spots on skin, redness, blistering, burning, tenderness, peeling, or loosening of skin or mucous membranes, red skin lesions often with a purple centre)

Some people may experience side effects other than those listed. Check with your doctor if you notice any symptom that worries you while you are taking Trimethoprim.

100 mg
Each white, round, biconvex tablet bisected and engraved "TRI" over "100" on one side contains 100 mg of trimethoprim. Nonmedicinal ingredients: colloidal silicon dioxide, croscarmellose sodium, magnesium stearate, and methylcellulose.

200 mg
Each yellow, round, biconvex tablet bisected and engraved "TRI" over "200" on one side contains 200 mg of trimethoprim. Nonmedicinal ingredients: colloidal silicon dioxide, croscarmellose sodium, magnesium stearate, methylcellulose, and D&C Yellow No. 10 Aluminum Lake 16%.

Do not take Trimethoprim if you:

• are allergic to trimethoprim or any ingredients of Trimethoprim
• are breast-feeding
• are pregnant
• have a blood condition known as megaloblastic Anemia (caused by folate deficiency)

There may be an interaction between trimethoprim and any of the following:

• amantadine
• Alfuzosin
• Amiodarone
• angiotensin-converting enzyme (ACE) inhibitors (e.g., Ramipril, Enalapril)
• angiotensin receptor blockers (e.g., ARBs; Irbesartan, Losartan, Valsartan)
• antipsychotics (e.g., Chlorpromazine, Clozapine, Haloperidol, Olanzapine, Quetiapine, Risperidone)
• Aprepitant
• Azathioprine
• bacillus Calmette-Guérin (BCG) vaccine
• Bosentan
• Carbamazepine
• Capecitabine
• Carvedilol
• Celecoxib
• chloroquine
• Dapsone
• Deferasirox
• Dexamethasone
• Diabetes medications (e.g., chlorpropamide, glipizide, Glyburide, insulin, Metformin, Nateglinide, Rosiglitazone)
• Digoxin
• Disopyramide
• dofetilide
• Domperidone
• Dronedarone
• Enzalutamide
• Eplerenone
• fenofibric acid
• Flecainide
• fluconazole
• Fluorouracil
• Gemfibrozil
• HIV non-nucleoside reverse transcriptase inhibitors (NNRTIs; delavirdine, efavirenz, etravirine)
• ketoconazole
• lamivudine
• leucovorin
• Leflunomide
• lopinavir
• macrolide antibiotics (e.g., clarithromycin, erythromycin)
• Memantine
• Mercaptopurine
• Methadone
• Methotrexate
• mifepristone
• Mitotane
• Modafinil
• Montelukast
• Omeprazole
• Paclitaxel
• Phenobarbital
• Phenytoin
• Pimozide
• Piroxicam
• primidone
• procainamide
• protein kinase inhibitors (e.g., Dabrafenib, Lapatinib, Nilotinib,
• Pazopanib, Sunitinib)
• quinidine
• quinine
• quinolone antibiotics (e.g., ciprofloxacin, Ofloxacin,)
• rifampin
• rilpivirine
• St. John's wort
• saquinavir
• selective serotonin reuptake inhibitors (SSRIs; e.g., Citalopram,  Fluoxetine, Paroxetine, Sertraline)
• serotonin antagonists (e.g.,  Granisetron, Ondansetron)
• siltuximab
• Sorafenib
• Sotalol
• Spironolactone
• sulfonamide antibiotics (e.g., sulfadiazine, sulfamethoxazole, sulfisoxazone)
• sulfonylureas (e.g., chlorpropamide, Gliclazide, glyburide, Tolbutamide)
• Tamoxifen
• Tetrabenazine
• tetrahydrocannabinol
• Ticagrelor
• Tocilizumab
• Trazodone
• tretinoin
• typhoid vaccine
• Varenicline
• Voriconazole
• Warfarin
• Zafirlukast
• Zopiclone

If you are taking any of these medications, speak with your doctor or pharmacist. Depending on your specific circumstances, your doctor may want you to:

• stop taking one of the medications,
• change one of the medications to another,
• change how you are taking one or both of the medications, or
• leave everything as is.

An interaction between two medications does not always mean that you must stop taking one of them. Speak to your doctor about how any drug interactions are being managed or should be managed.

Medications other than those listed above may interact with Trimethoprim. Tell your doctor or prescriber about all prescription, over-the-counter (non-prescription), and herbal medications you are taking. Also tell them about any supplements you take. Since caffeine, alcohol, the Nicotine from cigarettes, or street drugs can affect the action of many medications, you should let your prescriber know if you use them.

Before you begin using a medication, be sure to inform your doctor of any medical conditions or Allergies you may have, any medications you are taking, whether you are pregnant or breast-feeding, and any other significant facts about your health. These factors may affect how you should use Trimethoprim.

Blood disorders: The presence of sore throat, fever, pale skin, rash, or jaundice (yellowing of the skin) may be early signs of rare but serious blood disorders. If you develop any of these symptoms after starting trimethoprim, contact your doctor as soon as possible.

Folic acid levels: Trimethoprim affects how the body uses folic acid, a nutrient necessary for the formation of red blood cells. If you have a folic acid deficiency, discuss with your doctor how Trimethoprim may affect your medical condition, how your medical condition may affect the dosing and effectiveness of Trimethoprim, and whether any special monitoring is needed.

Kidney function: Kidney Disease or reduced kidney function may cause Trimethoprim to build up in the body, causing side effects. If you have reduced kidney function or kidney disease, discuss with your doctor how Trimethoprim may affect your medical condition, how your medical condition may affect the dosing and effectiveness of Trimethoprim, and whether any special monitoring is needed.

Liver function: Liver disease or reduced liver function may cause Trimethoprim to build up in the body, causing side effects. If you have reduced liver function or liver disease, discuss with your doctor how Trimethoprim may affect your medical condition, how your medical condition may affect the dosing and effectiveness of Trimethoprim, and whether any special monitoring is needed.

Pregnancy: When trimethoprim is taken during pregnancy, it crosses the placenta and may affect the unborn baby. Trimethoprim should not be used during pregnancy unless the benefits outweigh the risks. If you become pregnant while taking Trimethoprim, contact your doctor immediately.

Breast-feeding: Trimethoprim passes into breast milk. If you are a breast-feeding mother and are taking trimethoprim, it may affect your baby. Talk to your doctor about whether you should continue breast-feeding.

Children: The safety and effectiveness of using trimethoprim for infants younger than 2 months of age have not been established. The effectiveness of using trimethoprim alone (i.e., not in combination with other antibiotics) has not been established for children under 12 years of age.