Serious side effects from Phentermine are not common, but they can occur as your body adjusts to the new medicine. If you have any of the following symptoms in a severe form, or if they do not go away after a few days, call your doctor.

• dry mouth, or an unpleasant taste in your mouth
• bowel problems, such as diarrhea or constipation
• vomiting or other stomach upset

The symptoms listed below are more serious. If you have them, call your doctor right away:

• increase in your blood pressure
• palpitations (skipped beats) in your heartbeat
• restlessness
• dizziness
• tremors
• difficulty sleeping
• swelling in your legs or ankles

Sometimes, adjusting the dosage is enough to make side effects taper off or go away altogether.