Pioneers in the 1800s believed that breathing near a Gila monster could kill you. Today a drug modeled on a peptide in its venom is injected into millions of people. This episode separates the frontier legend from the animal: the largest native lizard in the United States, up to 22 inches long, armored with bony plates and patterned in pink, yellow, and black. It was named Heloderma suspectum in 1869 by Edward Drinker Cope, who suspected from its grooved teeth that it was venomous. Settlers went further and claimed it had no anus and exhaled poison gas.

The real lizard is a recluse. It spends about 90 percent of its life underground, stores fat in its tail and water in its bladder, and can live for a year on three or four meals of eggs and nestlings. Its venom is a defense, built to make a coyote let go, and it is chewed into a wound, not injected. In the early 1990s researchers found a venom peptide, exendin-4, that mimics the human hormone GLP-1 but resists the enzyme that destroys it. That discovery reshaped the treatment of diabetes and obesity.

• In 1891 Tombstone physician George Goodfellow paid five dollars a head for live specimens, let one bite his finger, spent five days in bed, and recovered.
• An adult can eat a third of its body weight in one meal, and a wild Gila monster may be active on the surface only 10 to 14 days a year.
• The venom glands sit in the lower jaw, and venom wicks up grooved teeth by capillary action while the lizard grinds its jaws.
• The FDA approved exenatide, a synthetic version of the lizard peptide sold as Byetta, in 2005, opening the way for the GLP-1 drugs.
• Arizona protected the species in 1952, and relocating one is often fatal because it burns its reserves trying to walk back to its burrow.