What are the possible side effects of phentermine?

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

itching.

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.