Get emergency medical help if you have signs of an allergic reaction to metronidazole (hives, itching, warmth or tingling; fever, joint pain; dry mouth, dry vagina; stuffy nose, difficult breathing, 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).
Call your doctor at once if you have:
new or worsening symptoms of infection;
painful or difficult urination;
confusion;
a light-headed feeling (like you might pass out); or
blisters or ulcers in your mouth, red or swollen gums, trouble swallowing.
Stop taking the medicine and call your doctor right away if you have neurologic side effects (more likely to occur while taking metronidazole long term):
numbness, tingling, or burning pain in your hands or feet;
vision problems, pain behind your eyes, seeing flashes of light;
muscle weakness, problems with speech or coordination;
trouble speaking or understanding what is said to you;
a seizure; or
fever, neck stiffness, and increased sensitivity to light.
Metronidazole can cause life-threatening liver problems in people with Cockayne syndrome. If you have this condition, stop taking this medicine and contact your doctor if you have signs of liver failure--nausea, stomach pain (upper right side), dark urine, clay-colored stools, or jaundice (yellowing of the skin or eyes).
Side effects may be more likely in older adults.
Common metronidazole side effects may include:
depression, trouble sleeping, feeling irritable;
headache, dizziness, weakness;
nausea, vomiting, loss of appetite, stomach pain;
diarrhea, constipation;
unpleasant metallic taste;
rash, itching;
vaginal itching or discharge, pain during sex;
mouth sores; or
swollen, red, or "hairy" tongue.
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.
 Metronidazole side effects (more detail)
