Regulators have ruled against Shire in a battle over generic copies of its hyperactivity drug Adderall XR, approving a cut-price version of the medicine from Actavis, which is being bought by Watson Pharmaceuticals.
The decision is a blow to Shire's near-term earnings and shares in the London-listed drugmaker tumbled 12 percent to 17.30 pounds by 5:40 a.m. EDT (0940 GMT).
Until now there have only been two authorized 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 on Monday it continued to believe it would deliver good full-year 2012 earnings growth.
Panmure Gordon, however, reduced its price target for the stock to 17.00 pounds from 17.80 previously and reiterated its "sell" recommendation.
Adderall XR has been a mainstay of Shire's expanding ADHD (attention deficit hyperactivity disorder) drug business, although the company has newer drugs for the condition in Vyvanse and Intuniv that are both increasing U.S. market share.
Mark Clark, an analyst at Deutsche Bank, said the impact of the arrival of Actavis' generic would be felt mainly on near-term earnings forecasts, since most Shire watchers had already assumed such generic competition in 2013.
Deutsche cut its 2012-13 earnings forecasts by 5-6 percent but it still expects attractive double-digit percentage growth over the medium term, with Shire's compound annual earnings growth rate put at 11 percent to 2015.
Shire has grown rapidly over the years through the successful marketing of drugs for ADHD. More recently, the company has diversified into new areas and now has a growing focus on rare diseases, an area of growing interest across the drugs sector.
The group has frequently been tipped as a takeover target for larger companies looking to replenish their drug pipelines.