- Nervous system side effects are among the most common side effects experienced by phentermine users. They are generally mild and begin to subside after the body adjusts to the drug. Phentermine patients should not take other appetite suppressants or weight loss drugs or supplements, as these increase the chance of more serious side effects. The most common nervous system side effects are anxiety, insomnia, restlessness, fatigue and headache.
- Gastrointestinal problems such as nausea, vomiting, constipation and diarrhea are common in phentermine patients. Some patients also complain of dry mouth or a persistent chemical-like taste. Gastrointestinal symptoms that arise as a result of phentermine use are generally mild and easily manageable, but patients should notify their physicians if gastrointestinal problems are severe or worsen over time.
- Phentermine usually increases heart rate. While not common, potentially serious cardiovascular side effects have occurred in some patients. According to WebMD, the probability of developing cardiovascular side effects increases in longer use. Chest pain, sudden high blood pressure, irregular heartbeats or an extremely fast heartbeat may be signs of serious cardiovascular damage or conditions. Patients experiencing any of these symptoms should stop taking phentermine and seek immediate medical attention.
- Psychiatric side effects are rare, but some phentermine patients have reported episodes of strange behavior, aggressive outbursts and hallucinations. Mild mood changes are more common mental side effects. Most patients who experience this side effect report a euphoric feeling while taking the drug. Changes in libido, or sex drive, are also relatively common. Some psychiatric drugs may increase the risk of mental side effects when taken with phentermine. Patients should discuss any psychiatric medications with their physicians before starting phentermine.
- Though rare, a small number of phentermine patients have reported serious and potentially fatal side effects and allergic reactions. Persistent tremors or shortness of breath may be signs of a severe reaction. Swelling of the face, mouth or extremities is not common and any occurrence of swelling a doctor should evaluate any occurrences of swelling in phentermine patients. A severe allergic reaction may cause hives, rash or severe itching. Patients experiencing any of these serious side effects or any cardiovascular side effects should seek immediate medical attention.
