Get emergency medical help if you have any of these signs of an allergic reaction: hives; difficulty breathing; swelling of your face, lips, tongue, or throat. Stop using phentermine and call your doctor at once if you have any of these serious side effects:
chest pain, swelling in your ankles or feet, feeling light-headed, fainting, or feeling short of breath, even with mild exertion;
fast or uneven heartbeat;
unusual thoughts or behavior, feeling restless or confused; or
increased blood pressure (severe headache, blurred vision, trouble concentrating, chest pain, numbness, seizure).
Less serious side effects may be more likely to occur, such as:
feeling nervous or anxious;
headache, dizziness, tremors;
trouble sleeping (insomnia);
dry mouth or an unpleasant taste in your mouth;
diarrhea or constipation; or
Side effects other than those listed here may also occur. Talk to your doctor about any side effect that seems unusual or that is especially bothersome.