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

More common:

• nausea and vomiting
• painful or swollen tongue;
• dizziness, spinning sensation
• ringing in your ears
• fatigue
• trouble sleeping
• increased sensitivity of the skin to sunlight

Call your doctor at once if you have a serious side effect such as:

• bloody diarrhea 
• fever, chills, swollen glands, body aches, flu symptoms, sores in your mouth and throat;
• new or worsening cough;
• pale skin, feeling light-headed, rapid heart rate, trouble concentrating;
• easy bruising, unusual bleeding (nose, mouth, vagina, or rectum), purple or red pinpoint spots under your skin;
• severe tingling or numbness, slow heart rate, weak pulse, muscle weakness;
• upper stomach pain, itching, loss of appetite, dark urine, clay-colored stools, jaundice (yellowing of the skin or eyes);
• urinating less than usual or not at all;
• skin rash
• severe skin reaction -- fever, sore throat, swelling in your face or tongue, burning in your eyes, skin pain, followed by a red or purple skin rash that spreads and causes blistering and peeling.

Sulfamethoxazole is an antibiotic in the sulfonamide family.  It prevents the formation of dihydrofolic acid, a compound that bacteria must be able to make in order to survive.  The effectiveness of sulfamethoxazole 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 it's antibiotic action. Infections caused by bacteria that are not sensitive to sulfamethoxazole will not show improvement after taking this medication.  Sulfamethoxazole is used primarily in combination with another antibiotic - trimethoprim- in products such as Bactrim® or Septra®.

Do not take sulfamethoxazole if you are allergic to sulfonamide drugs or "sulfa" drugs.  Allergic symptoms include wheezing, hives, itching, swelling, spasms in the throat and breathing tubes, joint and muscle pain, difficulty breathing, fever and skin rashes. Nausea and vomiting are not symptoms of an allergic reaction.
Taking the antibiotic repeatedly or for prolonged periods may result in bacterial or fungal overgrowth which can lead to a second infection. When this occurs, the sulfamethoxazole may need to be stopped and another medication prescribed to treat the new infection.
Diarrhea may develop while taking sulfamethoxazole.  Do not take any medicine to treat diarrhea without first checking with your doctor. If you have any questions or if mild diarrhea continues or gets worse, check with your doctor. 

Keep using this medicine for the full time of treatment, even if you begin to feel better after a few days. If you stop taking this medicine too soon, your symptoms may return.

Drug interactions:

• Angiotensin-converting enzyme (ACE) inhibitors (eg, enalapril), diuretics (eg, hydrochlorothiazide), or indomethacin - may increase the risk of sulfamethoxazole/trimethoprim's side effects
• Blood thinners (eg, warfarin) - risk of bleeding may be increased
• Dofetilide side effects, including severe irregular heartbeat, may be increased by sulfamethoxazole/trimethoprim
• Methenamine
• Amantadine, digoxin, phenytoin, methotrexate, sulfones (eg, dapsone), & oral medicine for diabetes (eg, glipizide, metformin, pioglitazone) - risk of their side effects may be increased by sulfamethoxazole/trimethoprim
• Tricyclic antidepressants (eg, amitriptyline) -their effectiveness may be decreased by sulfamethoxazole/trimethoprim
• Cyclosporine - effectiveness may be decreased and the risk of kidney side effects may be increased by sulfamethoxazole/trimethoprim

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

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