2007 has gotten off to a shaky start for pharmaceutical powerhouse Novartis (NYSE:NVS). In February, the Food and Drug Administration (FDA) issued an approvable letter delaying Novartis' diabetes treatment, Galvus, which was supposed to be its next blockbuster drug. On Friday, Novartis shareholders got another kick in the pants when the FDA requested that it pull Zelnorm, one of its top-selling drugs, from the U.S. market.

The FDA had reviewed past clinical trial data and noticed an "imbalance" of cardiovascular events occurring more often in patients taking Zelnorm, which is a treatment for irritable bowel syndrome. The FDA and Novartis agreed to pull the drug from the market until more information about the higher incidence of cardiovascular events is known.

Perhaps the FDA is just being conservative on this issue, since the percentage of patients treated with Zelnorm who had a cardiovascular event was just 0.11%, compared to .01% for those on placebo in the studies. But other drugs have been pulled in the past on similar increased cardiovascular statistical disparities.

Drug withdrawals from the market happen. The withdrawal can be at the FDA's urging, or done by pharmaceutical companies themselves if they think there is a chance the FDA will request the pulling (sort of like quitting a job before you know you'll get fired). The most publicized drug withdrawals happened in late 2004 with Merck's (NYSE:MRK) arthritis and pain drug Vioxx, and in early 2005 with Tysabri, Elan (NYSE:ELN) and Biogen Idec's (NASDAQ:BIIB) multiple sclerosis treatment. The FDA eventually allowed both drugs back onto the market, although Merck has chosen not to sell Vioxx anymore.

Even after a drug gets approved, there is always the chance that it can be pulled from the market if new safety issues pop up with its use. Due to the complexity and length of the clinical trial process and in getting a drug approved for marketing, the effect of many compounds over their long-term use is often unknown.

While there is still the chance that Zelnorm will be back on the market (and an FDA advisory committee will be set up to debate this), its sales potential will probably be kneecapped because of the negative publicity and the possibility of increased risk for cardiovascular events when using the compound.

Last year, Zelnorm was Novartis' 12th-highest selling drug with $560 million in sales, growing 34% over 2005's totals. For large pharmaceutical firms like Novartis, with hundreds of compounds on the market (including its generic division), setbacks like this will always be a reality. Investors should come to expect drug delays or similar issues popping up from time to time when doing their analysis of large pharmas.

Want intriguing growth-stock tips? Come take a 30-day pass to the Motley Fool Rule Breakers newsletter and see if it's right for you.

Fool contributor Brian Lawler got nicked shaving today and does not own shares of any company mentioned in this article. Biogen Idec is a Stock Advisor recommendation, and Merck is a former Income Investor selection. The Fool has a disclosure policy.