Glaxo(GSK) - Get Report stock slumped Wednesday as investors overlooked a positive set of first-quarter results to focus on the threat posed by generic competition to its blockbuster Advair asthma treatment.

The company warned again of the threat Wednesday although, since the patent for Advair expired in 2010, analysts have long waited with baited breath to see the impact generic competition will have on the Advair franchise, which is one of GSK's core drug lines.

Mylan (MYL) - Get Report , Hikma Pharmaceuticals (HKMPY) and Vectura (VEGPF) have generics in the pipeline. Mylan's generic was knocked back by the Food and Drug Administration in March but it might not be enough to keep competition from the market in 2017 as Hikma has said it could get approval for its own offering as soon as May.

The shares were more than 1% lower in London during afternoon trading, quoted at 1,584 pence, which marked a stark contrast with the near 1% gain of the Stoxx Europe 600 TMI Pharmaceuticals index.

Glaxo beat expectations for first-quarter earnings Wednesday and reiterated full-year guidance. But the catch is that the efficacy of this guidance depends on a market for Advair that remains generic free in 2017. Glaxo warned Wednesday that generics could rain on everybody's parade later this year.

GSK said earlier first-quarter earnings per share came in at £0.25 on an adjusted basis, modestly higher than the £0.24 figure forecast by analysts in a FactSet (FDS) - Get Report poll.

Revenues for the period hit £7.4 billion, beating the FactSet consensus of £7.249 billion, and ahead of the £6.23 Billion recorded in the first quarter of 2016. Advair sales were marked at £752 million for the three month period.

The company said it expects core earnings per share growth of around 5% to 7% in 2017 if there is no generic competitor to Advair released this year. If there is, GSK said core EPS would be flat or see a "slight decline in percentage terms" at constant exchange rates.