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What: Shares of drug developer Acorda Therapeutics (Nasdaq: ACOR) cratered today, falling as much as 19.1% from last night's closing price.

So what: European and Canadian regulators shocked Acorda owners by separately recommending against approval of multiple sclerosis drug Ampyra, raising questions about the pill's effectiveness weighed against the potential risks. The decision also damaged marketing partner Biogen Idec (Nasdaq: BIIB), though to a lesser degree because the larger company has more blockbuster irons in the fire than tiny Acorda does.

Now what: As an Ampyra user myself, I can attest to the symptom-modifying drug's effectiveness in at least one case. But anecdotal evidence won't do much to sway regulators, and the drug does have well-documented side effects. Acorda may have to go back to the lab again while leaning on American sales, but MS research continues unabated elsewhere – Novartis' (NYSE: NVS) disease-slowing pill Gilenya just got a thumbs-up from the same European agency that took exception to Ampyra, for example.

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