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

In 2017, the New Zealand Pharmaceutical Management Agency (PHARMAC) changed the funded version of the antidepressant venlafaxine from Efexor XR (the branded originator) or Arrow-Venlafaxine XR (a generic version) to a new generic, Enlafax XR. MacKrill and Petrie examined differences in preferences and perceptions between patients switched from the originator branded venlafaxine to the new generic, and those switched from the old generic version [1]. Specifically, they investigated the factors associated with preference for branded medicines, side effects reported following switching and efficacy ratings of the new generic.

Visitors to the venlafaxine brand change page on the PHARMAC website from March to October 2017 were invited via a link to complete a questionnaire about their experiences of the brand change.

A logistic regression was used to assess which factors are associated with a preference for branded medicines. In further analyses, the sample was separated into two groups: participants who switched from branded venlafaxine (brand switch group) and participants who switched from the old generic (generic switch group). Independent sample t-tests and χ2 tests were conducted to investigate whether there were any differences between the brand and generic switch groups in their perceptions about the efficacy, safety and side effects of branded and generic medicines, efficacy ratings of the new generic, or side effect reports. Multiple linear regressions were conducted to investigate the factors associated with greater side effect reporting and efficacy ratings of the new generic in the two groups. For all analyses, an alpha level of 0.05 was used.

The sample included 310 participants, comprising 205 previously taking branded venlafaxine and 105 previously taking a generic version. There were no significant differences in demographic variables between the two groups.

Preference for branded or generic medicine
In total, 180 participants (58.1%) reported a preference for branded medicines while 130 (41.9%) preferred generics or had no preference. Compared with people previously taking a generic, a higher proportion of those originally taking the branded venlafaxine considered branded medicines to be more effective, safer and have fewer side effects than generics. Those prescribed generics were more likely to perceive branded and generic medicines as equivalent.

The analysis of factors influencing people’s medicine preferences was significant, χ2 (12) = 44.74, p < 0.001, Nagelkerke R2 = 0.20. Preference for branded medicine was significantly higher in patients who had taken branded venlafaxine, older patients and patients with a higher perceived sensitivity to medicine.

Side effect reports for the new generic
Following the switch to the new generic, there was no significant difference between the brand switch group and generic switch group in reported side effects. Different factors predicted side effect reporting in the two groups.

The regression model of the brand switch group was significant, F(10, 172) = 3.61, p < 0.001, R2 = 0.17, and showed that participants who were older, female, had an education level below a university degree, had taken Efexor for a longer period of time and had a lower perceived efficacy of the new generic, reported a significantly greater number of side effects from the new generic. The regression model for the generic switch group was also significant F(10, 84) = 2.13, p = 0.031, R2 = 0.20, and showed that those who had a lower perceived efficacy of the new generic reported significantly more side effects following the switch.

Efficacy ratings of the new generic
Participants in both the brand and generic switch groups rated the efficacy of their previous medication very highly (brand switch group M = 8.46 out of 10, SD = 2.52; generic switch group M = 8.43, SD = 2.16). There were no significant differences in efficacy ratings of the new generic between the two groups.

The regression model was significant for the brand switch group, F(10, 172) = 3.72, p < 0.001, R2 = 0.18, and the generic switch group, F(10, 84) = 5.07, p < 0.001, R2 = 0.38. For both groups, a greater degree of trust in pharmaceutical agencies and fewer side effects were the only factors significantly associated with a greater perceived efficacy of the new generic drug.

This study highlights the fact that patients who switch from a branded medicine to a generic medicine are likely to have different concerns to those switching from a generic. Patients already taking a generic tend to have a more favourable perception of generics, while those taking a branded medicine tend to have more difficulties and concerns managing a generic switch.

Another important finding is the close reciprocal relationship between perceived drug efficacy and reported side effects. The presence of increased side effects is linked to a perception that the drug does not work as well and conversely, low efficacy beliefs are linked to increased reporting of side effects. In addition, the results show that trust in pharmaceutical agencies is a key factor in patients’ perceptions of the efficacy of a new generic medicine.

Conflict of interest
Keith J Petrie has previously received research funding from PHARMAC.

Editor’s comment
Readers interested to learn more about generics switching are invited to visit www.gabi-journal.net to view the following manuscript published in GaBI Journal:

Statin generics: no differences in efficacy after switching

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Reference
1. MacKrill K, Petrie KJ. What is associated with increased side effects and lower perceived efficacy following switching to a generic medicine? A New Zealand cross-sectional patient survey. BMJ Open. 2018;8:e023667. doi:10.1136/ bmjopen-2018-023667

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