Get emergency medical help if you have signs of an allergic reaction (hives, cough, chest pain, shortness of breath, swelling in your face or throat) or a severe skin reaction (fever, sore throat, burning eyes, skin pain, red or purple skin rash with blistering and peeling).
Seek medical treatment if you have a serious drug reaction that can affect many parts of your body. Symptoms may include: skin rash, fever, swollen glands, joint pain, muscle aches, severe weakness, pale skin, unusual bruising, or yellowing of your skin or eyes.
Call your doctor at once if you have:
severe stomach pain, diarrhea that is watery or bloody (even if it occurs months after your last dose);
a skin rash, no matter how mild;
yellowing of your skin or eyes;
a seizure;
new or unusual joint pain;
increased or decreased urination;
swelling, bruising, or irritation around the IV needle;
increased thirst, dry mouth, fruity breath odor;
new or worsening cough, fever, trouble breathing;
high potassium level--nausea, weakness, tingly feeling, chest pain, irregular heartbeats, loss of movement;
low sodium level --headache, confusion, slurred speech, severe weakness, vomiting, loss of coordination, feeling unsteady; or
low blood cell counts--fever, chills, mouth sores, skin sores, easy bruising, unusual bleeding, pale skin, cold hands and feet, feeling light-headed or short of breath.
Common side effects may include:
nausea, vomiting, loss of appetite; or
skin rash.
This is not a complete list of side effects and others may occur. Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088.
 Sulfamethoxazole/trimethoprim side effects (more detail)
