Common side effects of diltiazem include: flushing, headaches, vomiting, nasal congestion, constipation or diarrhea, cough and a slowed heartbeat. These side effects should be temporary, but if they do not go away let your doctor know. The most serious side effects of diltiazem include: a blistering skin rash, blood pressure low enough to cause dizziness, weakness or fainting (hypotension) and liver damage (jaundice). Get immediate medical attention if you experience these side effects. Other serious side effects include: flu-like symptoms, swelling of the hands, lower legs, ankles or feet, rapid weight gain, shortness of breath, loss of appetite or chest pain. Tell your doctor right away if you experience any of these symptoms.