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What is associated with increased side effects and lower perceived efficacy following switching to a generic medicine? A New Zealand cross-sectional patient survey

19 Oct 2018


Following a switch from either a generic or branded antidepressant (venlafaxine) to a new generic, we investigated the factors associated with a preference for branded medicines, side effects reported following switching and efficacy ratings of the new generic drug.


A cross-sectional survey of patients switched to a new generic.


Patients accessing venlafaxine information online from the New Zealand government pharmaceuticals funding website.


310 patients, comprising 205 originally on branded venlafaxine and 105 previously taking a generic version.

Main outcome measures

An online questionnaire assessing demographic factors, perceived sensitivity to medicines, trust in pharmaceutical agencies, sources of switch information, preference for branded medicine, new medicine perceptions, side effects and efficacy ratings.


Preference for branded medicine was significantly stronger in older patients (OR=1.04, 95% CI 1.01 to 1.05), those taking branded venlafaxine (OR=2.02, 95% CI 1.13 to 3.64) and patients with a higher perceived sensitivity to medicine (OR=1.23, 95% CI 1.06 to 1.19). Different factors predicted side effects in those switching from the branded and those switching from the generic venlafaxine. Trust in pharmaceutical agencies and the number of side effects were significant predictors of efficacy ratings of the new generic in both patients switching from a branded and those switching from a generic version of venlafaxine.


In patients switching from a branded medicine and those already taking a generic, different demographic and psychological factors are associated with preference for branded medicine, side effect reporting and perceived efficacy of the new drug. When switching to new generic, there appears to be a close bidirectional relationship between the experience of side effects and perceived drug efficacy. Trust in pharmaceutical agencies impacts directly on perceived efficacy and increasing such trust could reduce the nocebo response following a generic switch.

Click here to view the full article which appeared in BMJ Open