Potential drug-drug interactions with phentermine among long-term phentermine consumers: A retrospective analysis

Phentermine is an internationally recognised amphetamine derivative with significant appetite-suppressing properties. The drug is indicated for the short-term management of obesity, as the long-term (LT) use of phentermine may potentially be associated with severe cardiovascular side-effects, abuse and dependence. The LT use hereinafter describes periods exceeding 12 consecutive weeks. This use may also be associated with potential drug-drug interactions (PDDIs), which may result in adverse drug reactions (ADRs). The literature reports that phentermine is often prescribed LT and for several other off-label indications, increasing the risk for individuals to experience adverse drug events (ADEs) and drug-drug interactions (DDIs). There are, to our knowledge, no South African (SA) studies investigating the prevalence of co-prescribing LT phentermine with drugs that may potentially cause DDIs.

To determine the prevalence of mild, moderate and severe DDIs with phentermine use when the duration of therapy in private healthcare exceeded 12 consecutive weeks.

A cross-sectional drug utilisation review (DUR) was done by using data obtained from a SA pharmacy benefit management (PBM) company's database. Retrospective data of medicine claims for phentermine, from 1  January 2015 to 31  December 2019, were extracted for analysis. The number of days phentermine was supplied was used to identify the study population, in other words, those patients who received the drug LT. A drug interaction checker (Drugs.com) was used to identify potential mild, moderate and severe DDIs when using phentermine and co-prescribed drugs concurrently.

A total of 889 patients received phentermine LT. The top 20 drugs identified as being frequently co-prescribed in this study population demonstrated no mild PDDI, 15 (75%) moderate PDDIs and 5 (25%) severe PDDIs. The most common co-prescribed drug in the moderate group was dextromethorphan (n=282, 31.72%) and the least co-prescribed was formoterol (n=52, 5.85%). Among the drug group 'severe PDDIs', tramadol (n=416, 46.79%) was most frequently prescribed, whereas phenylpropanolamine (n=69, 7.76%) was the least prescribed to patients in this group.

There are patients who receive LT phentermine therapy despite the potential severe consequences that may result. These patients may receive concomitant therapy with phentermine and other pharmaceutical constituents, which may potentially cause DDIs, more specifically, moderate and severe DDIs. As such, these patients are not only confronted with the consequences of DDIs but are also at risk to experience ADRs as the residual effect of PDDIs.