Although we don't believe in timing the market or panicking over market movements, we do like to keep an eye on big changes -- just in case they're material to our investing thesis.

What: Shares of AcelRx Pharmaceuticals (NASDAQ:ACRX), a clinical-stage biopharmaceutical company focused on the treatment of acute pain, soared as much as 32% after the company reported positive top-line data from a late-stage trial. Shares have since given back nearly all of their gains, however, and are up only around 4% as of this writing.

So what: AcelRx announced before the opening bell that its pain management system, Sufentanil NanoTab PCA System, met its primary endpoint in a late-stage study of providing a greater reduction in post-operative pain as compared to the placebo. This study reconfirmed all of AcelRx's previously announced late-stage studies on its pain management system, and the company is on pace to file a new drug application by the third-quarter.

Now what: It's certainly a positive day for AcelRx shareholders, but I'm not nearly as excited about today's data. I'm not disputing that AcelRx's drug delivery system met its primary endpoint. Instead, I'm concerned about another opioid substance going before the Food and Drug Administration. Opioid substances don't have a particularly good track record of approval with the FDA, and AcelRx has already had a huge run higher in anticipation of today's data release. There's only so much blood you can squeeze out of a turnip, and I foresee considerably more chance of downside than upside at current levels.

Craving more input? Start by adding AcelRx Pharmaceuticals to your free and personalized watchlist so you can keep up on the latest news with the company.

Fool contributor Sean Williams has no material interest in any companies mentioned in this article. You can follow him on CAPS under the screen name TMFUltraLong, track every pick he makes under the screen name TrackUltraLong, and check him out on Twitter, where he goes by the handle @TMFUltraLong.

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