From Lexi-comp for Dentistry:
Phentermine
Local Anesthetic/Vasoconstrictor Precautions: Use vasoconstrictor with caution in patients taking phentermine. Amphetamines enhance the sympathomimetic response of epinephrine and norepinephrine leading to potential hypertension and cardiotoxicity.
Effects on Dental Treatment/Key adverse event(s) related to dental treatment: Xerostomia (normal salivary flow resumes upon discontinuation) and unpleasant taste. Up to 10% of patients may present with hypertension. The use of local anesthetic without vasoconstrictor is recommended in these patients.
Dental Health Professional Considerations: Many diet physicians have prescribed fenfluramine ("fen") and phentermine ("phen"). When taken together the combination is known as "fen-phen". The diet drug dexfenfluramine (Redux®) is chemically similar to fenfluramine (Pondimin®) and was also used in combination with phentermine called "Redux-phen". While each of the three drugs alone had approval from the FDA for sale in the treatment of obesity, neither combination had an official approval. The use of the combinations in the treatment of obesity was considered an "off-label" use. Reports in medical literature have been accumulating for some years about significant side effects associated with fenfluramine and dexfenfluramine. In 1997, the manufacturers, at the urging of the FDA, agreed to voluntarily withdraw the drugs from the market. The action was based on findings from physicians who evaluated patients taking fenfluramine and dexfenfluramine with echocardiograms. The findings indicated that approximately 30% of patients had abnormal echocardiograms, even though they had no symptoms. This was a much higher than expected percentage of abnormal test results. This conclusion was based on a sample of 291 patients examined by five different physicians. Under normal conditions, fewer than 1% of patients would be expected to show signs of heart valve disease. The findings suggested that fenfluramine and dexfenfluramine were the likely cause of heart valve problems of the type that promoted FDA's earlier warnings concerning "fen-phen". The earlier warning included the following: The mitral valve and other valves in the heart are damaged by a strange white coating and allow blood to flow back, causing heart muscle damage. In several cases, valve replacement surgery has been done. As a rule, the person must, thereafter for life, be on a blood thinner to prevent clots from the mechanical valve. This type of valve damage had only been seen before in persons who were exposed to large amounts of serotonin. The fenfluramine increases the availability of serotonin.
