Cephalon (NASDAQ:CEPH) shares have been on a tear, with a 70% gain since September alone. Even a soft showing in recently reported fourth-quarter earnings failed to dampen enthusiasm for the drug developer. While the company admittedly has had some good news of late, investors should be careful that they don't get ahead of themselves.

For last year's fourth quarter, Cephalon indicated that revenue rose 13% to $336.4 million, while earnings per share came in at $0.30, a drop of almost 75.6% from $1.23 in the fourth quarter of 2004. Admittedly, the bottom-line drop was exacerbated by a gaggle of one-time items. Even when taking these into account, though, adjusted EPS still declined 13% to $0.71 from $0.82.

The market seems more concerned, though, with Cephalon's good news. For one thing, the patent for the firm's top seller, Provigil, a medicine for excessive sleepiness arising from certain sleep disorders, is safe from generic competition until 2011 thanks to a deal with generic outfits Mylan Laboratories (NYSE:MYL) and Teva Pharmaceutical (NASDAQ:TEVA), among others.

However, Cephalon enthusiasts also appear to be pinning their hopes on Sparlon, a medicine for ADHD under review at the FDA. Sparlon's active ingredient is the same as that in Provigil, so chances for approval seem good.

What's more, some are speculating that Sparlon could benefit from an FDA panel's recent recommendation to put black box warnings on some marketed ADHD medicines because of the potential risk of heart attack, stroke, and sudden death. Since Sparlon is not technically a stimulant like many of the marketed drugs, the reasoning is that it could draw patients switching from other ADHD meds.

While it's probably true that Sparlon would gain from the black box warning, the likelihood of a large-scale exodus is less evident. The FDA panel was divided on the warning, as was clear from the 8-to-7 vote count. And it's easy to see why they would be -- the decision was based on the sudden deaths of 25 people on ADHD drugs from 1999 to 2003. Considering an estimated 4 million people regularly use ADHD medications, the risk seems relatively small.

Consequently, Sparlon may not take off as quickly as some expect. That doesn't mean Cephalon is a bad company -- far from it. Still, investors may want to keep their enthusiasm for Sparlon in check.

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Fool contributor Brian Gorman is a freelance writer in Chicago. He does not own shares of any companies mentioned in this article.