Human Genome Sciences (Nasdaq: HGSI) and GlaxoSmithKline (NYSE: GSK) will likely get a decision about their lupus drug Benlysta this week. The decision was delayed three months, so investors can only hope that the Food and Drug Administration will make its goal of a decision by March 10.

Benlysta has made it a lot farther than Roche and Biogen Idec's (Nasdaq: BIIB) Rituxan, BioMarin Pharmaceutical's (Nasdaq: BMRN) Riquent, Teva Pharmaceutical's (Nasdaq: TEVA) edratide, and so many others that died in clinical development. Can it get past the finish line to be the first new drug designed to treat lupus in more than 50 years? I think so.

Yes, Benlysta is far from perfect. The robust effect seen after one year of treatment seems to wane, and the drug works better in some populations than others. But the unmet need -- 50 years is a long time to wait -- should help Benlysta overcome its shortcomings. It passed two phase 3 trials. An advisory panel overwhelmingly recommended approving the drug. That's all you can ask for from a drug company developing a lupus drug.

Assuming the FDA approves the drug, the most pressing issue will be the label the agency gives Benlysta. The label tells doctors exactly who the drug is approved to treat and provides supporting material like clinical trial data.

Benlysta wasn't tested in patients with lupus that attacks the kidneys or the central nervous system, so those patients will likely be excluded as potential recipients.

More worryingly, the drug didn't work particularly well in African American patients. If the FDA recommends not using it in that population, or even highlights the fact on the label, the companies could lose up to 25% of the potential market.

The typical binary FDA decision event has become a sea of potential outcomes. It's not out of the question that Benlysta gets approved, but Human Genome Sciences shares fall because the drug label will dampen sales.

BioMarin Pharmaceutical is a Motley Fool Rule Breakers selection. GlaxoSmithKline is a Motley Fool Global Gains pick. The Fool owns shares of GlaxoSmithKline and Teva Pharmaceutical Industries. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors.

Fool contributor Brian Orelli, Ph.D., doesn't own shares of any company mentioned in this article. The Fool has a disclosure policy.