GlaxoSmithKline shares fell the most in 20 months Monday after U.S. regulators proposed a simpler route for rivals seeking to copy the company’s Advair drug, threatening almost $8 billion in revenue.

Glaxo (NYSE: GSK) declined as much as 3.6 percent, the biggest intraday drop since Jan. 9, 2012, and was down 3.1 percent in early trading Tuesday in London.

Shares closed Monday in New York at $50.46, down $1.21 or 2.3 percent.

In pre-market trading Tuesday, shares were down another 1.35 percent or 68 cents to $49.78.

The Food and Drug Administration on Monday revised guidelines for competitors to get copies on the market, saying long, expensive human tests aren’t required to show drugs work the same as Advair. The inhaler is the world’s third-best selling medicine with revenue of 5.05 billion pounds ($7.9 billion) and accounts for about 20 percent of London-based Glaxo’s annual sales.

“The vast majority of analysts have assumed that true generic versions of Advair would not likely launch in the U.S.,” Tim Anderson, an analyst at Sanford C. Bernstein & Co. said in a note to clients after the FDA statement. “This new draft guidance document suggests otherwise.”

Glaxo, and until now the FDA, have said that the same drug in a different inhaler is a different product because the device can cause dosing to vary. While the patent on Advair expired in 2010 in the U.S., a separate patent on the Diskus device used to administer the medicine remains in force through 2016.

Novartis AG’s generics division, Sandoz, probably will be the first in getting an Advair copy to the market, followed by Actavis Inc. and Mylan Inc., according to Bernstein.

“With respiratory drugs it’s difficult to know how much is actually getting into your lungs,” said Mark Purcell, an analyst at Barclays Plc in London. “So the FDA was trying to reconcile that. It’s a big step forward for other companies not to have to come up with complicated experimental ways to try to prove equivalence.”

The FDA said it’s reviewing issues raised in Glaxo’s 2009 citizen petition and will consider comments on draft bioequivalence recommendations before responding to the petition.

GSK maintains its North American headquarters in RTP.

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