Serious side effects of ACTONEL may include: allergic reaction including hives, difficulty breathing, swelling of your face, lips, tongue, or throat., chest pain, difficulty or pain when swallowing, pain or burning under the ribs or in the back, new or worsening heartburn, severe or ongoing indigestion, severe joint, bone, or muscle pain, new or unusual pain in your thigh or hip, jaw pain, numbness, or swelling flu symptoms, muscle pain, mild stomach pain or upset stomach, diarrhea, constipation, mild joint or back pain, headache