GlaxoSmithKline (GSK - Get Report)  beat analysts' expectations for fourth-quarter sales and earnings Wednesday but warned of a significant hit this year from anticipated generic competition to its Advair asthma treatment.  

The drugmaker reported sales of £7.5 billion ($9.3 billion) in the three months ending in December, up 21% on the year before, and beating the Factset consensus for £7.48 billion. Earnings per share were 26.1 pence, an increase of 45%, and a beat against the consensus for EPS of 24 pence.

"2016 has seen GSK perform strongly with good sales growth across all three businesses, excellent new product momentum, disciplined cost control and further pipeline progress," said CEO Andrew Witty. 

The company said that in 2017, earnings could face a considerable hit due to the anticipated introduction of generic rivals to Glaxo's Advair asthma treatment in the U.S. 

Glaxo shares were marked 1.6% lower at 1,536 pence each in London following the release, taking them to a three-month loss of nearly 3%.

Mylan (MYL - Get Report) , Hikma Pharmaceuticals (HKMPY and Vectura (VEGPF all have generics pending approval with the FDA, while Mylan could get the go-ahead to market its own as soon as March.

"Clearly, this year we face some uncertainty as to the level of our earnings performance, given the possibility of substitutable generic competition to Advair in the U.S., and this is reflected in the guidance we have issued today," added Witty.

Glaxo's anticipates earnings per share growth of between 5% and 7% in the event that no generic competition to Advair makes it to market during 2017. However, if Mylan or any other outfit gains approval in time for a mid-year launch, then the group has warned that it could finish the year with earnings per share having fallen by a low-single-digit number.

For Glaxo, 2016 was a year of mixed blessings. It benefited greatly from the U.K.'s vote to leave the European Union, given the subsequent devaluation of sterling which has since boosted earnings.

But it has been challenged by a growing line of competitors all threatening to encroach on its blockbuster asthma treatment. Returns for the shares were also tempered by the election of Donald Trump to the U.S presidency. Trump fought his campaign and entered the White House on a pledge to take a tough stance against drug companies on pricing. Investors have grown concerned that Glaxo and others could soon find it more difficult to raise prices for treatments sold in the U.S.