Background: Predictions of drug sales play an important role in setting drug prices in Japan, where drug prices are highly regulated. One of 2 primary Japanese drug-pricing methodologies—the cost calculation method— refers extensively to the prediction of drug sales in establishing drug prices. Deviations from predicted sales can induce disproportionate drug prices and unexpected pharmaceutical expenditures. This study aimed to estimate the upward deviation of actual sales from predicted sales and to explore the predictors of such upward deviation. Methods: Estimates of upward deviation in 2015 were produced for new molecular entities (NMEs) approved in 2006-2015. To identify the predictors associated with upward deviation through binary logistic regression analyses, sales within 3 years of launch and in the predicted year in which peak sales would be reached were used. Results: Estimated upward deviation was more than 1220 billion yen in 2015 for the targeted drugs. Drugs priced by the cost calculation method or by obtaining additional indications were significantly more likely to show an upward deviation from predicted peak sales. Conclusion: There is substantial upward deviation between actual and predicted drug sales in Japan. So long as drug sales predictions are used in drug price calculations, a flexible repricing system is needed to buffer unexpected pharmaceutical expenditures.
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