The hereditary angioedema market is teeming with options. Last week, the FDA approved Shire Pharmaceuticals' (Nasdaq: SHPGY) Firazyr to treat the rare genetic disease.

Patients with hereditary angioedema get attacks of extreme swelling on parts of their body. If the attack occurs near the neck, it can be deadly by cutting off the ability to breathe.

Treating attacks
Firazyr was approved to treat the attacks as they occur. It can be self-administered, which is a clear advantage over CSL's Berinert and Dyax's (Nasdaq: DYAX) Kalbitor, which both have to be administered by a doctor.

While hereditary angioedema is a fairly small indication -- less than 30,000 Americans have the disease -- there's still money to be made. BioMarin Pharmaceuticals (Nasdaq: BMRN), Alexion Pharmaceutical (Nasdaq: ALXN), and Novartis have gone after diseases that affect fewer people because they can make up in price what they lose in volume. Dyax, for instance, is guiding for $20 million to $24 million in Kalbitor sales this year, even though it had only set up 545 patients to receive the drug through the end of June.

Preventing them in the first place
ViroPharma (Nasdaq: VPHM) has a hereditary angioedema drug that's approved to prevent attacks rather than treat them once they occur. Like Shire's drug, Cinryze is self-administered. It's an infusion, but only requires 10 minutes every three to four days. In order to make life even easier for the patient, ViroPharma is developing a version that can be delivered via a subcutaneous shot using technology from Halozyme Therapeutics (Nasdaq: HALO).

In the meantime, the infusion version seems to be selling well. Sales of Cinryze totaled $119 million in the first half of the year, and ViroPharma expects that will continue to grow. Management is guiding for sales between $131 and $141 million in the second half of the year, which would put the company exiting the year with a run rate above $280 million annually.

Finding patients
Is there enough room for four products -- and perhaps a fifth that's being developed by Pharming and Santarus (Nasdaq: SNTS)? Maybe.

On one hand, divvying up 30,000 patients isn't nearly as lucrative as capturing all of them. A small slice of a personal-sized pizza isn't very filling.

But there's a weird phenomenon in orphan drug indications where the markets tend to grow when more drugs get approved. Orphan diseases are often under-diagnosed, so if you double -- or quadruple, in this case -- the sales force talking with doctors about the disease, they may be more apt to diagnose the indication.

The added choices can work on the patient-demand side, too. When the first drug comes on the market, patients that have lived their entire lives without a treatment may be leery of taking the medication. But after seeing other patients doing well on the drug, they may be more inclined to talk to their doctor about taking a drug. The patients serve as a positive feedback loop, and the more drugs available, the more intense the feedback will be.

Which one?
I think the two drugs most likely to succeed are Shire's Firazyr, because it's more convenient, and ViroPharma's Cinryze, because prophylactics usually sell better than acute treatments.

But neither is a pure play on hereditary angioedema. Shire sells other orphan drugs and treatments for attention-deficit disorder; ViroPharma sells an antibiotic called Vancocin that's been under threat of generic competition for many years, but still contributes a significant chunk of ViroPharma's revenue. It also has a seizure product, Buccolam, which should be approved in the EU shortly.

I'll leave you to do your own due diligence on the other drugs. You can keep up on how the companies are doing in the hereditary angioedema market by adding them to the Fool's free My Watchlist service.