AVANIR Pharmaceuticals'(Nasdaq: AVNR) investors didn't have any uncontrolled crying on Friday. The Food and Drug Administration approved Nuedexta, the company's treatment for pseudobulbar affect (PBA). 

They may however be laughing (all the way to the bank) today. AVANIR's shares more than doubled today on the good news.

An overnight double is usually a sign that investors were expecting a rejection, but in this case, I think the low price ahead of the FDA decision was simply a product of investors' uncertainty over which way the FDA would rule -- this Fool included.

An earlier version of the drug was rejected by the FDA because of potential heart problems. AVANIR reduced the concentration of the offending component of the drug, which seemed to help alleviate the problem. But given the FDA's propensity towards safety -- Eli Lilly (NYSE: LLY) and Amylin Pharmaceuticals' (Nasdaq: AMLN) Bydureon was delayed for the same potential issue -- investors were right to be worried that AVANIR's data wouldn't be enough to sway the FDA.

Nuedexta is the only drug approved to treat PBA, a disease characterized by uncontrolled laughing and/or crying due to underlying neurological disease such as multiple sclerosis or amyotrophic lateral sclerosis (ALS).

Much like Acorda Therapeutics' (Nasdaq: ACOR) Ampyra, which treats multiple sclerosis patients who are unable to walk, Nuedexta has the advantage of not having to compete with established drugs such as Teva Pharmaceuticals' (Nasdaq: TEVA) Copaxone and Biogen Idec's (Nasdaq: BIIB) Avonex. Instead, it will be used in combination with those drugs, which seek to slow down the progression of the disease.

How big is the PBA market? Since there aren't any drugs approved to treat PBA and only a subset of patients with neurological diseases get PBA, it's a little hard to know how large the market really is.

Still with a market cap under $500 million, investors still haven't priced many sales into the stock. If the launch goes well next year, there could be another double -- albeit not an overnight one -- in AVANIR's future.

Interested in keeping track of Avanir as it launches Nuedexta? Click here to add it to My Watchlist, which will help you keep track of all our Foolish analysis on AVANIR.

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