A retrospective analysis of phentermine in the private healthcare sector of South Africa : duration and drug-drug interactions

Abstract

Background: Phentermine is the preferred appetite suppressant amongst South African healthcare prescribers. Guidelines are available to assist prescribers in rational and safe prescribing of the drug. The long-term (LT) use of phentermine possibly linked to amphetamine-like side effects justifies why the duration of use thereof is, as specified in the prescribing guidelines, limited to a period not surpassing 12 consecutive weeks. These guidelines also refer to certain drugs phentermine should not be co-prescribed with as it may result in mild, moderate or even severe drug-drug interactions (DDIs) and thus causing adverse drug reactions (ADRs). Phentermine continues to be prescribed LT and for other indications, consequently increasing the risk of adverse drug events (ADEs) and DDIs. Objectives: To analyse the duration of therapy (DOT) with phentermine (15 mg and 30 mg respectively) and to determine the prevalence of potential drug-drug interactions (PDDIs) amongst LT phentermine consumers in private healthcare. Method: A quantitative, retrospective analyses was conducted using approved and paid claims data for phentermine from 1 January 2015 to 31 December 2019. Information of the age, gender and prescriber’s speciality of patients receiving 15 mg and 30 mg phentermine were considered and evaluated. The total number of consecutive days 15 mg and 30 mg phentermine was supplied for, respectively, was used to determine the DOT and to identify the patient group that received phentermine LT. Drugs co-prescribed with phentermine were extracted and the top 20 drugs, which may have caused PDDIs, most frequently prescribed to LT phentermine consumers were listed accordingly. These PDDIs were categorised according to severity as mild, moderate or severe, using a drug interaction checker from Drugs.com. Results: There were 3 361 patients receiving phentermine over the study period of which 2 848 (84.74%) patients were female and 35-59 years of age (n = 1 952; 58.08%). Patients mostly received phentermine short-term (ST) (n = 2 472; 73.55%) and 889 (26.45%) patients received the drug LT. The average DOT with 15 mg phentermine was 57.26 (58.72) days and with 30 mg phentermine it was 67.10 (52.01) days (p < 0.001; d-value = 0.17). The DOT with 15 mg and 30 mg phentermine varied from 5-720 days and 1-360 days, respectively.

The top 20 drugs most frequently co-prescribed with phentermine, amongst LT consumers, demonstrated no mild PDDIs. There were 15 (75%) drugs which may have possibly caused moderate DDIs; with dextromethorphan most frequently (n = 282; 31.72%) and formoterol (n = 52; 5.85%) least prescribed to patients in this group. The residual 25% of drugs may have potentially caused severe DDIs and tramadol (n = 416; 46.79%) was most frequently prescribed to patients. The drug least prescribed in this group was phenylpropanolamine (n = 69; 7.76%). Conclusion: In South Africa, the DOT with phentermine, in private healthcare, is not restricted to ST use. Patients receive LT therapy and are therefore exposed to the risk of experiencing amphetamine-like side effects, PDDIs and ADRs. Prescribers should use prescribing guidelines when prescribing phentermine as it eliminates irrational drug prescribing and prevents PDDIs and ADRs.