Shares in Canada's Biovail plunged yesterday after it emerged that its Wellbutrin XL antidepressant would soon face generic competition.

A judge in a Toronto court ruled that although Biovail's patents on the sustained release formulation of GSK's bupropion generic were valid, a copycat version developed by Anchen Pharmaceuticals did not infringe upon them.

Biovail gets an approximately 30% royalty rate on Wellbutrin XL sales and the drug accounts for 40% of the firm's total revenues.Anchen will be able to launch its version as soon as it received approval from the US Food and Drug Administration, although Biovail is likely to appeal the ruling.

In a statement, Biovail said it is reassessing its financial guidance for 2006 of revenue between $900 million and $1.07 billion and share earnings per share of $2.30 to $2.40, which observers said suggested it was now expecting to face competition to its flagship product sooner rather than later.

Biovail also has a Citizen's Petition filed with the FDA asking for "rigorous bioequivalence testing" for Wellbutrin XL generics before they can be approved, which if granted could also delay the launch of Anchen's rival product.