Get emergency medical help if you have signs of an allergic reaction to phentermine: hives; difficult breathing; swelling of your face, lips, tongue, or throat.
Call your doctor at once if you have:
feeling short of breath, even with mild exertion;
chest pain, feeling like you might pass out;
swelling in your ankles or feet;
pounding heartbeats or fluttering in your chest;
tremors, feeling restless, trouble sleeping;
unusual changes in mood or behavior; or
increased blood pressure - severe headache, blurred vision, pounding in your neck or ears, anxiety, nosebleed.
Common phentermine side effects may include:
itching;
dizziness, headache;
dry mouth, unpleasant taste;
diarrhea, constipation, stomach pain; or
increased or decreased interest in sex.
This is not a complete list of side effects and others may occur. Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088.
 Phentermine side effects (more detail)
