Venomous lizards exist: the Gila monster and its chewing bite

If the Gila monster kept a calendar, most days would read “underground.” The Smithsonian’s National Zoo says it spends most of its life below the surface, with most of its surface activity packed into about three spring months. A KJZZ report says wild Gila monsters spend up to 95% of their time underground. Then, on the rare days it appears, this lizard raises a question most lizards never do: are there venomous lizards?

There are, and the Gila monster (Heloderma suspectum) is the best-known example. It is also a lesson in how many ways an animal can be venomous. A snake’s venom glands sit in its upper jaw, and a Gila monster’s sit in its lower jaw. A rattlesnake injects through hollow fangs, and a Gila monster chews. This article covers where the venom comes from, how it gets into a bite, what it does, and how it ended up as a medicine.

This article is not a guide to handling Gila monsters. They are protected by law and should be watched from a distance.

Meet the Gila monster

The Gila monster lives in the deserts and dry rocky country of the southwestern United States and northwestern Mexico. The name comes from the Gila River. It is stout and slow, with a thick tail, and the Animal Diversity Web describes its scales as beaded yellow, pink and black.

The Reptile Database lists two named forms, a banded one and a reticulated (net-patterned) one. It also notes that two later genetic studies found no mitochondrial DNA support for separating them, though poor records of where the samples came from make the results hard to judge.

How many venomous lizards are there?

It depends on who is counting. The Animal Diversity Web calls the Gila monster one of only two venomous lizard species, the other being the Mexican beaded lizard. The Reptile Database labels it one of the few venomous lizards. That is the traditional answer, and the venom delivery system of these two lizards is described in detail in zoology and medical sources.

A different answer appeared in 2005. A Penn State news release described a Nature paper led by Bryan Fry of the University of Melbourne, arguing that venom evolved much earlier than thought, about 200 million years ago by S. Blair Hedges’s estimate. The release said some lizards once thought harmless also produce venom, including some large monitors, the family of the Komodo dragon. The release also quoted one of the researchers as expecting the scientific community to take a few years to accept the new family tree, and we did not find a newer source on how that debate ended. So the honest answer is that two lizards are firmly known for it, and others are debated.

For how snake venom evolved, see the King cobra.

Glands in the lower jaw, delivery by chewing

Grzimek’s Animal Life Encyclopedia, in an entry by Daniel Beck, says the Gila monster’s venom glands are in the lower jaw, unlike a snake’s, which sit in the upper jaw. The glands have several lobes and empty through ducts at the base of grooved teeth. David Warrell’s medical chapter describes bulging glands in the front of the lower jaw, where the venom pools along the lips, and says the venom reaches the wound mainly through the grooved lower teeth, especially the fourth to seventh. It adds that the lizard drools venom-rich saliva when threatened.

The grooves do the work that a hollow fang does in a snake. The National Zoo says venom is drawn along them by capillary action, the same effect that lets a paper towel soak up water from its corner. The lizard bites, holds on, and chews, and the Zoo says it can hang on for more than ten minutes. A small amount of venom enters per bite, according to the Zoo, though the same page says the venom is about as toxic as a western diamondback’s. That is the odd bargain of the Gila monster: strong venom, slow delivery.

For the heat-sensing, fang-injecting approach, see the Western diamondback rattlesnake.

What a bite does, and why the venom is mostly for defense

Grzimek’s entry says the venom is used mainly for defense, and the Animal Diversity Web notes that prey are rarely envenomated. Their diet, mostly eggs, nestlings and other young animals, is food that does not fight back. The sources do not link those two facts, so that connection is our observation, not a finding.

Bites to people are rare. Warrell’s chapter says they are virtually never accidental, because the lizards are reclusive and not aggressive, and that nearly all bites land on the fingers, hands or forearms of people who were handling or catching one. Not every bite delivers venom. When it does, he describes severe throbbing or burning pain that often lasts 24 hours or more, rapid swelling, and early signs of dizziness, weakness, nausea, vomiting and sweating, with a drop in blood pressure common. No antivenom is generally available, according to Warrell and the Zoo, so treatment supports the patient, and recovery is generally expected in under a week.

Deaths are very rare. Beck’s entry says no human death has been reported since 1930, and Warrell calls the few earlier alleged deaths difficult to attribute. A 2024 KJZZ report described a death in Colorado after a bite from a pet, and put the previous known bite death around 1930. KJZZ also notes that keeping one without a permit is illegal in Arizona. Anyone bitten needs medical care.

From venom to medicine

Venom is a chemical toolbox, and sometimes a tool turns out to be useful. The Golden Goose Award says Dr. John Eng, working at a Veterans Administration hospital in the Bronx, read NIH studies in the late 1980s on how snake and lizard venoms affect the pancreas. In 1992 he found a compound in Gila monster venom and named it exendin-4. It stimulates insulin-producing cells to make more insulin when blood sugar is high, which the award page says lowers the risk of blood sugar dropping too low compared with an insulin shot. A drug that grew out of it, exenatide, was approved by the FDA in 2005 and is sold as Byetta. Beck’s entry also lists exendin-4 among peptides found in the venom in the 1990s.

It is rare for a lizard bite and a pharmacy shelf to share a story. Here they do.

Common mistakes, myths and mix-ups

Myth: lizards cannot be venomous. The Gila monster and its Mexican relative are the classic exceptions, and some researchers argue there are more.

Mix-up: venomous and poisonous mean the same thing. Warrell’s chapter separates them: envenoming means venom is injected through the skin or applied to mucous membranes, while poisoning comes from eating or touching something toxic. A Gila monster is venomous.

Wrong picture: it injects venom like a rattlesnake. There are no hollow fangs. The venom is chewed in along grooves.

Not so: it stalks and attacks people. Warrell describes the lizards as reclusive and non-aggressive, and bites as virtually never accidental.

Mix-up: Gila monster and Mexican beaded lizard are one animal. They are two species. Warrell gives the Gila monster up to about 55 cm in total length and the beaded lizard up to about 1 meter, and The Reptile Database separates them by tail length and other body details.

Overstated: venomous means deadly. The medical sources describe painful, sometimes serious bites, but deaths are exceptionally rare.

Conservation status

The Gila monster is listed as Near Threatened on the IUCN Red List of Threatened Species. That is the category as distributed by GBIF, the Global Biodiversity Information Facility, and we checked it on November 9, 2026. Near Threatened means the species was evaluated and does not currently meet the criteria for a threatened category, but is close to qualifying or may qualify in the near future.

The sources name several pressures. The National Zoo cites habitat destruction from overgrazing, truck farming and cotton planting. KJZZ quotes an Arizona State University researcher naming roads and home building as the biggest threats. The Animal Diversity Web adds illegal capture, a small pet trade, and killings by people who think the lizards are dangerous. Beck’s entry mentions collectors. On the positive side, the Animal Diversity Web says the lizard is legally protected in every state where it lives, and an Arizona Game and Fish Department manager quoted by KJZZ says it is not endangered and is common in some areas. Beck’s entry in Grzimek’s encyclopedia says the IUCN lists both helodermatid species as Vulnerable, which differs from the GBIF copy. The page we read does not show the entry’s date, so it may be older. We have no confirmed population trend, because none of our sources states one.

Categories are reviewed and can change, and the copy GBIF distributes can trail the IUCN website by some months, which is why we print the date we checked. GBIF’s copy does not give the year of the assessment, so we do not state one.