Omalizumab (Xolair) can cause side effects, ranging from mild to serious. Common side effects include fatigue, muscle or joint aches, injection site reactions, stomach pain, headache, skin rash, dizziness, bone fractures, ear pain or infections, upper respiratory infections, nasal symptoms, nosebleeds, and nausea. Serious side effects may include severe allergic reactions, shortness of breath, fever, rash, muscle aches, swollen glands, blood vessel inflammation, parasitic infection, cancer, and heart or circulation issues. If side effects persist or are severe, contact your doctor. For serious symptoms like severe allergic reactions or shortness of breath, seek immediate medical help.