When drug developer Acorda Therapeutics (NASDAQ:ACOR) released positive phase 3 results for its multiple sclerosis (MS) treatment earlier in the year, shares skyrocketed. They have been up more than 600% since their lows for the year, seen right before the positive trial results.

On Friday, Acorda updated investors on the progress of its MS drug, named Fampridine-SR. As expected, the FDA is requiring another phase 3 trial of the drug before Acorda can file a new drug application for marketing approval. Interestingly enough, despite some worrying adverse events and the chronic nature of MS (which means patients need to be on the drug for the long term), this second phase 3 study is expected to be no longer than the first short, 14-week phase 3 trial in which Fampridine-SR was tested; it also has the same simple objective of improving patients' timed 25-feet walk.

Additionally, a second trial will be run to test Fampridine-SR's effect on the heart's functioning. This safety study should be watched closely, since Fampridine-SR is a potassium channel blocker. Many potassium channel blockers have been associated with adverse effects on the heart, although in trials to date Fampridine-SR has been safe in this regard.

I fully expect Fampridine-SR to show solid efficacy results in this upcoming phase 3 trial. The shorter-acting version of the drug has been tested for its effect on neurological functioning since the 1970s and has shown effectiveness in multiple clinical trials. The question that remains is whether Fampridine-SR will be deemed safe enough to get approval even with positive efficacy results.

Even if approval happens on the first go-round (which I doubt), I envision Fampridine-SR as having a very restrictive label, considering the drug only improves symptoms related to MS and does not inhibit the progression of the disease. Also, it is associated with some bad side effects.

When retail investors hold a high proportion of shares of a small specialty pharma company like Acorda, the stock price is liable to be mighty volatile. With such a long time to wait before any potential approval of Fampridine-SR, I expect this volatility to continue. More risk-adverse investors would be smart to stay away.

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