Many new drugs initially enjoy banner sales growth, only to get kneecapped by reports of previously unknown adverse side effects. On Friday, the FDA issued an alert "highlighting" a stricter warning label for one of Pfizer's (NYSE: PFE) most important new drugs.

The compound in question, Chantix, is one of the most recent non-nicotine stop-smoking treatments on the market. The FDA approved it back in 2006, but after its launch, some of the more than 5 million patients who have tried the drug began to report adverse events, including anxiety, other serious psychiatric changes, and "vivid" and "unusual" dreams. As a result of these reports, Pfizer strengthened the warning label for Chantix. Last week's FDA announcement formalized more specific safety warnings for the drug's label.

As with GlaxoSmithKline's (NYSE: GSK) Wellbutrin, one of Chantix's biggest advantages against Nicoderm and most other smoking-cessation agents is that it contains no addictive nicotine. During its 2007 year-end earnings conference call last month, Pfizer's executives didn't reveal what portion of the potential market for Chantix has a psychiatric illness that would prevent it from using the drug. If Pfizer can't pin down the patients most at risk for such psychiatric problems in tandem with Chantix, the drug's market opportunity will diminish. More doctors will prescribe it only as a backup treatment, in cases when other anti-smoking therapies don't work.

Chantix is Pfizer's most important new drug right now. Sales of the drug have grown much faster than I expected, up a very strong 312% year over year in the fourth quarter, to $280 million worldwide. Besides Chantix, and a revenue boost from pain drug Lyrica after its fibromyalgia label expansion last June, Pfizer has few other new growth opportunities on the market or in its late-stage drug pipeline.

Fortunately for investors, the 2008 guidance Pfizer issued two weeks ago already accounted for the effects of this labeling revision (and a similar one pending in the European Union) on this year's revenue. Still, if Chantix's excellent sales trajectory stalls, the tough situation Pfizer faces in coming years will only get more difficult.

GlaxoSmithKline is an active Income Investor pick. Pfizer is an active Inside Value pick.

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