A pharmaco-economic analysis of patients with schizophrenia switching to generic risperidone involving a possible compliance loss

Background: As schizophrenia patients are typically suspicious of,
or are hostile to changes they may be reluctant to accept generic
substitution, possibly affecting compliance. This may counteract
drug costs savings due to less symptom control and increased
hospitalization risk. Although compliance losses following generic
substitution have not been quantified so far, one can estimate the
possible health-economic consequences. The current study aims to do
so by considering the case of risperidone in Germany. Methods: An
existing DES model was adapted to compare staying on branded
risperidone with generic substitution. Differences include the
probability of non-compliance and medication costs. Incremental
probability of non-compliance after generic substitution was varied
between 2.5% and 10%, while generic medication costs were assumed
to be 40% lower. Effect of medication price was assessed as well as
the effect of applying compliance losses to all treatment settings.
The probability of staying on branded risperidone being
cost-effective was calculated for various outcomes of a
hypothetical study that would investigate non-compliance following
generic substitution of risperidone. Results: If the incremental
probability of non-compliance after generic substitution is 2.5%,
5.0%, 7.5% and 10% respectively, incremental effects of staying on
branded risperidone are 0.004, 0.007, 0.011 and 0.015 Quality
Adjusted Life Years (QALYs). Incremental costs are (sic)757,
(sic)343, -(sic)123 and -(sic)554 respectively. Benefits of staying
on branded risperidone include improved symptom control and fewer
hospitalizations. If generic substitution results in a 5.2% higher
probability of non-compliance, the model predicts staying on
branded risperidone to be cost-effective (NICE threshold of 30,000
per QALY gained). Compliance losses of more than 6.9% makes branded
risperidone the dominant alternative. Results are sensitive to the
locations at which compliance loss is applied and the price of
generic risperidone. The probability that staying on branded
risperidone is cost-effective would increase with larger compliance
differences and more patients included in the hypothetical study.
Conclusion: The model predicts that it is cost-effective to keep a
patient with schizophrenia in Germany on branded risperidone
instead of switching him/her to generic risperidone (assuming a 40%
reduction in medication costs), if the incremental probability of
becoming non-compliant after generic substitution exceeds 5.2%..