Side effects. The side effects of Botox injections are usually mild and include redness and swelling at the injection sites that can be eased by ice packs. More serious Botox side effects include allergic reactions that can include a rash and itching. Some people get a headache, pain in their neck or back or muscle stiffness. Other side effects are shortness of breath, diarrhea, nausea, abdominal pain, loss of appetite, muscle weakness, flu-like or cold-like symptoms such as runny nose, sore throat, cough and fever. Some people experience bleeding or infection at their injection sites. Some people become dizzy, tired or anxious. Others have a dry mouth, tinnitus and problems with urination.