Evaluating reasons for US low generics substitution rates

The uptake of generic drug products in the US has not met expectations despite their widespread availability and relatively low cost. A team of researchers based at Johns Hopkins University in the US, have now investigated the reasons for generics utilization and substitution across therapeutic classes. They reveal marked variation in reasons, with women less likely to substitute for generics, those using mail order pharmacies less likely to substitute, and state specific laws having little influence.

Many studies have explored the reasons for the relatively low uptake of generics in the US, focusing on opinion surveys of patients, clinicians and pharmacists [1, 2]. They reveal the continued concern over biosimilar bioequivalence, safety and efficacy across the US. However, this recent investigation has looked beyond opinions and explored the effects of insurance plan design, the type of dispensing pharmacies, and state-level policy, on substitution levels.

In 2012, the US Generic Drug User Fee Amendments of 2012 (GDUFA) led to accelerated generics approval. In addition, the patents and exclusivity periods of many originator products have recently expired. These factors have led to increased availability of generics products in the US in recent years. In 2016, almost 90% of all prescriptions were for generics, yet they accounted for just over a quarter of the cost spent on medicines due to the comparative high prices of originator products.

The team conducted a cross-sectional analysis of data from the 2013 MarketScan Commercial Claims and Encounters Database from Truven Health Analytics on claims from individuals (and their spouses and dependants) with employer-sponsored private health insurance. They then determined generics substitution rates for 26 drug classes, during one representative week in November 2013.

Results
The data revealed that over 5.5 million unique medications of interest, within 26 therapeutic classes (made up of 219 unique drugs), had been prescribed to 4.2 million patients during the week of interest. Eighteen of the 26 therapeutic classes had generics substitution rates of 90% or above which meant that eight classes underwent further examination by the team. These were: thyroid hormones, oestrogens and combinations, anticonvulsants (hydantoin), androgens and combinations, Immunosuppressants, parathyroid hormones (calcitonin, spray), ocular preparations for pressure lowering, and stimulants (amphetamine type).

Following statistical analysis, it was possible to estimate the independent relationship between the determinants of interest and generics substitution for eight classes where generics utilization was low. The reasons varied markedly across therapeutic classes. For example:

In five of the eight classes, younger age predicted generics utilization, whereas in the remaining three classes older age predicted generics utilization.
Men were more likely to fill generics prescriptions in six of the seven tested classes, with women more likely to fill for generic immunosuppressants.
Patients with a higher medication burden were more likely to fill for a generic drug product for six of the eight studied classes.

In addition, the impact of insurance type on generics substitution was also variable. Patients with an insurance product other than fee-for-service were markedly less likely to use generic ophthalmic drugs. However, those in high-deductible health plans were more likely than the fee-for-service beneficiaries to use generics in three classes: anticoagulants, stimulants and thyroid hormones.

When looking at retail and mail order pharmacy fills, the use of mail order pharmacy was associated with less generics substitution in five of the eight classes (stimulants, thyroid hormone drugs, anticoagulants, ophthalmic drugs and oestrogens).

The study also revealed that state laws had markedly inconsistent effects across the drug classes. Contrary to expectations, in five of the eight classes, filling in a state that requires patient consent for substitution raised the likelihood of generics substitution. Additionally, laws about mandatory or permissive substitution of generics by pharmacists had very little impact on the generics substitution rates. However, where mandatory substitution was not required the use of generic ophthalmic drugs and thyroid hormones was reduced.

Conclusions
In conclusion, the study authors note that the determinants of generic drug use vary across drug classes but there are important patterns emerging. They recommend that stakeholders committed to getting affordable medications to patients should think about focussing on specific classes with substantially low generics substitution rates. They can then design targeted interventions according to the observed determinants. Additionally, further investigation of the impact of mail order pharmacies, or the role of pharmacy benefit managers, is needed to achieve the aim of increased generics usage across the US.

Factors associated with increased side effects and lower perceived efficacy when switching to a generic

References
1. GaBI Online - Generics and Biosimilars Initiative. Pharmacists prefer generic OTC medicines [www.gabionline.net]. Mol, Belgium: Pro Pharma Communications International; [cited 2020 Sep 11]. Available from: www.gabionline.net/Generics/Research/Pharmacists-prefer-generic-OTC-medicines
2. Shrank WH, Cox ER, Fischer MA, et al. Patients’ perceptions of generic medications. Health Aff (Millwood). 2009;28(2):546-56.

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