Shares in Indian drugmaker Dr Reddy's have lost almost one-fifth of their value, after the firm lost a crucial patent case in the US.
Pfizer sells $4bn-worth of Norvasc every year
A US court on Friday banned Dr Reddy's from selling a cheap generic version of Norvasc, a popular anti-hypertension drug made by Pfizer.
The ruling overturns a previous court decision, and is a severe blow for the Indian firm's business model.
Low-cost Indian drugmakers see generic copies as their main growth area.
Dr Reddy's and Indian rivals such as Cipla and Ranbaxy want to copy drugs still under patent, because they offer far more generous
Ups and downs
Anyone is allowed to copy a drug whose patent has lapsed, but margins there tend to be modest.
Dr Reddy's argues that its generic drugs do not violate the intellectual property of companies such as Pfizer, since they are careful to ensure that their copies are never exact.
The firm is the most aggressive of its peers, with three-quarters of its pending US patent applications based on challenges to existing intellectual property.
But the practice has run into stiff opposition from major pharmaceutical companies, which say they are entitled to reap the rewards of their own research.
They have been backed up by regulators, especially in the US.
Meanwhile, US consumers are becoming ever more vocal about the high prices of patented medicines, something that has led to a boom in internet and black-market sales.