Shares dropped 11.34 per cent after the approval of a rival cut-price version of the medicine from Actavis, which is being bought by Watson Pharmaceuticals.
Adderall XR has been a mainstay of Shire’s ADHD (attention deficit hyperactivity disorder) drug unit.
Until now there have only been two authorised generic forms of Adderall XR, both supplied by Shire, and the arrival of the new competitor from Actavis, despite an appeal against the move by Shire, will increase pricing pressure.
Analysts at Jefferies said the Food and Drug Administration’s approval of the first abbreviated new drug application had been anticipated, but the move had come sooner than expected.
Despite the setback, Shire said yesterday it continued to believe it would deliver good full-year 2012 earnings growth.
Shire has grown rapidly over the years via the successful marketing of drugs for ADHD. More recently, the firm has diversified into new areas with a focus on rare diseases.