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LiverTox: Clinical and Research Information on Drug-Induced Liver Injury [Internet]. Bethesda (MD): National Institute of Diabetes and Digestive and Kidney Diseases; 2012-.

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LiverTox: Clinical and Research Information on Drug-Induced Liver Injury [Internet].

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Last Update: June 4, 2020.



Phentermine is a sympathomimetic amine and anorectic agent used for the short term therapy of obesity. Phentermine which has been in clinical use for more than 50 years has not been linked convincingly to either serum enzyme elevations during therapy or to instances of clinically apparent liver injury.


Phentermine (fen' ter meen) is a structural analogue to amphetamine and has similar activity in suppressing appetite, but has few of the other central nervous system effects of amphetamines and only mild abuse potential. Its effect in increasing weight loss is probably mediated by anorectic activity that is the result of enhancement of norepinephrine release in appetite centers in the brain. Phentermine was approved as a therapy for obesity in the United States in 1959 but was recommended only for short term use (less than 12 weeks) and in combination with behavioral modification, caloric restriction and exercise. Nevertheless, phentermine is widely used with more than 2 million prescriptions filled in the United States yearly. Phentermine is a Schedule IV drug, meaning that it has proven, but low abuse potential and has an accepted medical use. Phentermine is available by prescription as 30 mg tablets of phentermine base and 37.5 mg tablets of phentermine hydrochloride in multiple generic forms and under various trade names including Adipex-P, Obenix and Lonamin. The usual dose is 30 or 37.5 mg once daily. A combination of phentermine with topiramate (an anticonvulsant that has weight loss effects) has recently been approved for use for long term use in patients who are obese or are overweight and have obesity related conditions. This combination consists of fixed lower doses of phentermine and topiramate (3.75/23, 7/46, 11.25/69 and 15/92 mg) and is available in capsules under the brand name Qsymia. Common side effects of phentermine include nervousness, excitability, insomnia, headache, dry mouth, sweating, tachycardia, palpitations, nausea, constipation, and thirst. Rare severe adverse events include atrial fibrillation, acute psychosis and pulmonary hypertension.


Phentermine has not been linked to an increased rate of serum enzyme elevations during therapy; however, results of ALT monitoring during phentermine therapy have rarely been reported. Despite long term availability and wide use of phentermine, there have been no published reports linking it to clinically apparent acute liver injury.

Likelihood score: E (unlikely cause of clinically apparent liver injury).

Drug Class: Weight Loss Agents, see also Phentermine-Topiramate



Phentermine – Generic, Adipex-P®, Lonamin®


Weight Loss Agents


Product labeling at DailyMed, National Library of Medicine, NIH


Phentermine 122-09-8 C10-H15-N image 134973544 in the ncbi pubchem database


References updated: 06 June 2020

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    (Expert review of hepatotoxicity published in 1999; phentermine is not discussed).
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