Normally any delay in getting FDA approval to market a new drug is considered an unmitigated negative. But when Cephalon (NASDAQ:CEPH) announced on Friday that the agency issued a second approvable letter for wakefulness drug Nuvigil, it was a case of no big deal.

Nuvigil is a follow-on version of Cephalon's compound Provigil, which brings in $700 million a year in sales. Provigil accounted for over 40% of Cephalon's revenue last year and is slated to face generic competition no later than 2012. Depending on the outcome of some lawsuits, it could be even sooner.

Cephalon expects to submit a response to this second Nuvigil approvable letter and to receive a much less onerous Class 1 review for the drug, which pushes possible approval of Nuvigil out to around July.

Since Provigil has several years of patent protection left on it, getting Nuvigil onto the market immediately is not as big a priority at this point, considering that the drug will gain much of its sales growth from the cannibalization of Provigil sales. What's more important is that the FDA is not asking for any more clinical trial work with Nuvigil before granting it the marketing go-ahead. 

Nuvigil will give Cephalon patent protection on at least part of its wakefulness franchise products until 2023. Cephalon also expects to expand the market opportunity for Nuvigil beyond its initial indications by testing it in new patient groups, starting this year.

As the fortunes of Nuvigil and Provigil go, so goes Cephalon's stock price. Now that the approval of Nuvigil is almost a sure thing, shares of Cephalon are more attractive for investors. But with this year's stagnating sales -- due to the recent generic competition for one of its other products -- investors must be willing to wait.

Fool contributor Brian Lawler does not own shares of any company mentioned in this article. The Fool has a disclosure policy.