Abbott Laboratories' (NYSE: ABT) Humira is like the little engine that could. It keeps chugging along, picking up more approvals and growing revenues. And that's all well and good, as long as Congress doesn't place a steep downhill run in its path.

Abbott's most recent score for Humira came in the form of an FDA approval to treat moderate to severe chronic plaque psoriasis -- a skin disease. That's the fifth FDA approval for the anti-inflammatory, which is also approved to treat two forms of arthritis, Crohn's disease, and a spinal inflammation called ankylosing spondylitis.

Humira is entering a crowded psoriasis market, including Amgen's (Nasdaq: AMGN) and Wyeth's (NYSE: WYE) Enbrel and Genentech's (NYSE: DNA) Raptiva, among others. Still, Abbott thinks the newest approval can eventually add $500 million in revenue, atop the $3 billion in sales for Humira's other approved uses.

Abbott's not done with the label expansions either. The drugmaker is testing Humira as a treatment for ulcerative colitis, a form of inflammatory bowel disease (IBD).

As long as biologics continue to have de facto infinite marketing exclusivity, Abbott's strategy of label expansion is a good one. While the clinical trials are expensive to run, Abbott can easily recoup the cost, as long as there's no generic competition.

On the other hand, if Congress ever gets around to passing legislation allowing the FDA to approve follow-on biologics, sales of Humira will be perched on a pretty high mountain, with only one direction to go. At least the drugmaker seems to have a backup plan.

More Foolishness on label expansions:

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