Get emergency medical help if you have signs of an allergic reaction to clarithromycin: (hives, difficult breathing, swelling in your face or throat) or a severe skin reaction (fever, sore throat, burning in your eyes, skin pain, red or purple skin rash that spreads and causes blistering and peeling).
Call your doctor at once if you have:
severe stomach pain, diarrhea that is watery or bloody;
fast or pounding heartbeats, fluttering in your chest, shortness of breath, and sudden dizziness (like you might pass out);
confusion, spinning sensation;
liver problems - loss of appetite, upper stomach pain, tiredness, dark urine, clay-colored stools, jaundice (yellowing of the skin or eyes); or
kidney problems - little or no urination, swelling in your feet or ankles, feeling tired or short of breath.
If you've ever had heart problems, clarithromycin may cause a heart attack or stroke years after you've taken this medicine. Seek medical attention right away if you have chest pain spreading to your jaw or shoulder, trouble breathing, slurred speech, or sudden numbness or weakness on one side of your body.
Older adults may be more likely to have heart rhythm side effects, including a life-threatening fast heart rate.
Common clarithromycin side effects may include:
stomach pain, nausea, vomiting;
diarrhea; or
unusual or unpleasant taste in your mouth.
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.
 Clarithromycin side effects (more detail)
