Phentermine may cause some serious side effects, including heart and lung problems. You and your doctor should discuss the good this medicine may do as well as the risks of taking it.

Along with its needed effects, Phentermine may cause some unwanted effects. Although not all of these side effects may occur, if they do occur they may need medical attention.

Check with your doctor immediately if any of the following side effects occur:
Chest pain; decreased ability to exercise; fainting; swelling of feet or lower legs; trouble in breathing

Check with your doctor as soon as possible if any of the following side effects occur:

Increased blood pressure, difficult or painful urination; fast or irregular heartbeat; feeling that others can hear your thoughts; feeling that others are watching you or controlling your behavior; hallucinations (feeling, seeing, or hearing things that are not there); headache (severe); mental depression; numbness, especially on one side of the face or body; skin rash or hives; sore throat and fever (with diethylpropion); talking, feeling, and acting with excitement and activity you cannot control; unusual bleeding or bruising (with diethylpropion)

Symptoms of overdose

Abdominal or stomach cramps; coma; confusion; convulsions (seizures); diarrhea (severe); dizziness, lightheadedness, or fainting; fast breathing; feeling of panic; fever; hallucinations (seeing, hearing or feeling things that are not there); high or low blood pressure; hostility with urge to attack; irregular heartbeat; nausea or vomiting (severe); overactive reflexes; restlessness; trembling or shaking; tiredness, weakness, and mental depression following effects of excitement

Abuse of a sympathomimetic appetite suppressant (taking the medicine in larger doses or taking it more frequently or for a longer time than the doctor ordered) can cause the following side effects:
Changes in personality; excessive, excited activity; irritability (severe); mental illness (severe), similar to schizophrenia; skin disease; trouble in sleeping (severe)

Other side effects may occur that usually do not need medical attention. These side effects may go away during treatment as your body adjusts to the medicine. However, check with your doctor if any of the following side effects continue or are bothersome:

Constipation; dizziness or lightheadedness; dryness of mouth; false sense of well-being; headache; irritability; nausea or vomiting; nervousness or restlessness; stomach cramps or pain; trembling or shaking; trouble in sleeping;

Blurred vision; changes in sexual desire or decreased sexual ability; diarrhea; drowsiness; frequent urge to urinate or increased urination; increased sweating; unpleasant taste;