Some investors say it's best to wait until after regulatory decisions to buy and sell biotech stocks. As Genzyme (NASDAQ:GENZ) showed yesterday, this can be a tough investing strategy to implement because a biopharma's stock will often move unpredictably on news.

The news you would have thought would have dropped shares of Genzyme yesterday was that the FDA issued an approvable letter for a second-generation improved version of its Synvisc osteoarthritis pain reliever.

Synvisc, which essentially acts as a lubricant for the knees of patients that have lost their natural joint fluid, is one of Genzyme's most important products. Sales of the first-generation Synvisc were $234 million last year and accounted for 7% of Genzyme's revenue. In its just released third-quarter earnings, sales of the drug were up 9% year over year.

In order to better compete against similar lubricants marketed by Johnson & Johnson (NYSE:JNJ) and Sanofi-Aventis (NYSE:SNY), Genzyme's second-generation Synvisc was supposed to provide the same level of pain relief as the original, but require only one visit to the doctor, rather than the three visits currently required.

Genzyme didn't give any indication what FDA concerns prompted the approvable letter, except the very generic statement that the agency was requesting "additional analyses and data," which could mean anything from needing to run another clinical trial to just having to report data already on hand.

Now, instead of a first-half 2008 launch of its improved formulation of Synvisc, the new timeline is that the drug will be delayed "until at least the second half of 2008."

This forecast for the launch likely means that a new clinical trial of some sort needs to be run. But since Genzyme already is generating sales from the original Synvisc, the extra wait for the new formulation shouldn't materially harm its top-line growth forecasts much (if at all).

Sure, the 3.2% rise in shares of Genzyme yesterday assuredly had more to do with the huge run-up in the broader markets, but it just goes to show that trying to time a biotech buy or sell order on the news often won't work.

More Foolishness for your achy joints:

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