There's a fine balance to be struck with multiple sclerosis drugs and other drugs that modulate the immune system, like Johnson & Johnson's (NYSE:JNJ) and Schering-Plough's (NYSE:SGP) rheumatoid arthritis drug, Remicade. The drugs have to stop the body from attacking itself, but not shut down the immune system too much -- it still has to be there to fight outside infections.

That's the issue that Novartis (NYSE:NVS) is having with its new multiple sclerosis drug candidate, FTY720. The drug could be a game-changer, but investors should wait for results next year before they pencil in blockbuster revenue from it.

On Friday, Novartis announced promising results from a phase 3 trial on FTY720, which sounds more like a skateboard move than a potential blockbuster drug. The drug reduced the relapse rate by 38% and 52%, compared with Biogen Idec's (NASDAQ:BIIB) Avonex at two different doses. Considering that Avonex brought in $1.6 billion in the first nine months of the year, we're talking about some serious potential revenue.

But the trial was only a year long, and FTY720 has had some issues with safety, including two fatal herpes infections, along with seven incidences of skin cancer compared with just one in the group taking Avonex. Data from two longer-term two-year trials are expected next year, and those studies are likely to determine how successful FTY720 is on the market.

Much has been made about FTY720 being administered orally versus the current drugs that are injected, like Avonex, Teva Pharmaceutical's (NASDAQ:TEVA) Copaxone, and Rebif, from EMD Serono and Pfizer (NYSE:PFE). While it's true that the drug could take a substantial piece of the market from the injected drugs, that's only going to happen if FTY720 proves to be at least as effective -- so far, so good -- with no worse side effects in the long term -- still a big unknown. If it can't do that, it'll be relegated to a second-line drug, as Biogen and Elan's (NYSE:ELN) Tysabri has mostly become.

Novartis plans to apply for marketing authorization next year. If FTY720 can stick its landing and find the sweet spot of balancing the benefits and side effects, the drug could be a blockbuster. Keep an eye on this one, Fools.