Phentermine and other monoamine-oxidase inhibitors may increase plasma serotonin when given with fenfluramines
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TL;DR
It is shown that phentermine inhibits the MAO which catabolises serotonin, which was well known in the early 1970s, and the finding that plasma serotonin concentration fell slightly was shown.
Abstract
Phentermine has been used in the USA to treat obesity since at least 1961 and was widely used after 1992 in combination with fenfluramine. A study in 1997 suggested that this combination could be associated with heart-valve disease similar to that seen in the carcinoid syndrome or in patients who had taken ergotamine, as well as with pulmonary hypertension, previously described. 1 Abenhaim L Moride Y Brenot F et al. Appetite-suppressant drugs and the risk of primary pulmonary hypertension. N Engl J Med. 1996; 335: 609-615 Crossref PubMed Scopus (1110) Google Scholar It was suggested that these findings, were due to an increase in circulating serotonin. Subsequently, the US Food and Drug Administration found heart-valve lesions among patients taking a fenfluramine without phentermine.