Get emergency medical help if you have signs of an allergic reaction: hives; difficulty breathing; swelling of your face, lips, tongue, or throat.
In rare cases, this medicine can cause a condition that results in the breakdown of skeletal muscle tissue, leading to kidney failure. Call your doctor right away if you have unexplained muscle pain, tenderness, or weakness especially if you also have fever, unusual tiredness, or dark colored urine.
Also call your doctor at once if you have:
a light-headed feeling, like you might pass out;
eye pain, vision problems;
an unusual skin rash;
easy bruising, unusual bleeding;
jaundice (yellowing of the skin or eyes);
kidney problems--little or no urination, painful or difficult urination, swelling in your feet or ankles, feeling tired or short of breath;
low levels of sodium in the body--headache, confusion, slurred speech, severe weakness, vomiting, loss of coordination, feeling unsteady;
high potassium--slow or unusual heart rate, weakness, loss of movement; or
low potassium--leg cramps, constipation, irregular heartbeats, fluttering in your chest, increased thirst or urination, numbness or tingling, muscle weakness or limp feeling.
Common side effects include:
nausea;
dizziness;
cold symptoms such as stuffy nose, sneezing, sore throat; or
high levels of uric acid in your blood.
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.
 Hydrochlorothiazide/olmesartan side effects (more detail)
