Most of the reptiles that arrive at a vet with bent legs, a soft jaw or a shell like rubber were not sick in the usual sense. Their owners fed them and kept them warm, and the animals dissolved their own bones anyway, because a lizard or tortoise cannot use the calcium in its food without vitamin D3, and most of them cannot make vitamin D3 without ultraviolet light. This page is that chain, the signs to watch for, and the rules the veterinary bodies give for lamps, dusting and gut-loading, with the places they disagree left in.
The vets call it nutritional secondary hyperparathyroidism; everyone else calls it metabolic bone disease or MBD. Merck's professional manual describes it as "the most common bone disease encountered in reptile practice", and its pet-owner edition gives the cause in one sentence: "a poor diet with the wrong ratio of calcium to phosphorus and a lack of vitamin D3 or poor husbandry (lack of ultraviolet B light, inadequate temperature control in the cage)". The University of Illinois explains the mechanism: when the blood runs short of calcium, the parathyroid gland tells the body to take it out of the skeleton, and the bones are broken down to keep the nerves and muscles running. Calcium "is required for nerve transmission, muscle function, blood clotting and a multitude of other vital bodily functions", so the body will spend the skeleton to protect them.
The chain has three links, and every one of them has to hold. Ultraviolet B light of the right wavelength turns a cholesterol in the skin into vitamin D3, and warmth finishes the conversion. Vitamin D3 lets the gut absorb calcium from food. And the food has to contain calcium in the first place, in more than it contains phosphorus, which most insects and many vegetables do not. Merck's nutrition page says the calcium-to-phosphorus ratio of most foods offered to reptiles "is inadequate and should be at least 1:1, with 2:1 preferred", and its table gives mealworms 0.06 percent calcium and crickets 0.23, which is why the insect eaters get it most.
The early ones are quiet. LafeberVet's client handout says affected animals "frequently exhibit signs of weakness or lethargy such as reluctance to move, decreased appetite, and weight loss". Then the bone signs: a lower jaw that swells or goes soft ("rubber jaw"), swollen back legs, legs that tremble when the animal tries to walk, a lizard that crouches flat because it cannot push itself up, a spine that bends, a limb with too many angles in it because it has broken, and in turtles a shell that is soft or uneven. Washington State's list ends with seizures in advanced cases. RSPCA's sentence is the one to remember: "muscle twitching, swollen legs, fragile bones and eventually, permanently deformed limbs, jaws, spine or tail".
Who gets it: Merck says "rapidly growing herbivorous and insectivorous lizards and chelonians", so young bearded dragons, leopard geckos, chameleons, iguanas and tortoises above all. VCA says it is most often seen in bearded dragons under two years old, and calls it "probably the most common medical problem of pet iguanas".
Often, if it is caught while the animal is still eating. Merck's professional line is "The prognosis is favorable, particularly if the animal is still eating", and correcting the diet and the lighting is "the mainstays of successful therapy" alongside fluids, calcium and, in bad cases, injected vitamin D3 or calcitonin. The honest version of the outcome comes from a Chicago exotics practice: "Many pets recover completely. Some are left with permanent deformities or nervous deficits. Some will die." Deformed bones do not straighten. While an animal is being treated, LafeberVet's handout says to handle it "as if they are made of porcelain".
The bulb has to emit UVB, which is light between about 290 and 320 nanometers; ordinary bulbs, "daylight" bulbs and heat lamps do not. It has to be close: Merck's professional manual says a fluorescent UVB lamp must be "relatively close to the reptile (< 30 cm [12 inches])", VCA says 9 to 15 inches for its benefit to reach the animal, and Merck's pet-owner page allows up to 18 inches, with mercury-vapor lamps able to work from further away. It has to be replaced while it still looks fine: UVB output falls long before the visible light does, so Merck says every 9 to 12 months, VCA 9 to 12 months or as the maker says, LafeberVet's bearded dragon handout every six months, and Washington State every six to eight months for a growing animal and yearly for an adult. And nothing can sit between the bulb and the animal: "Most transparent plastic and glass barriers filter out UVB wavelengths" (Merck), which is also why a tank by a sunny window gets no useful UV at all. A UV index meter, the same instrument the RSPCA tells owners to use, is the only way to know what a lamp is actually delivering.
Nocturnal and dusk-active species are not exempt. VCA says that although they "may not openly bask, they too increase their vitamin D concentrations following UV exposure", and Merck says all reptiles "including nocturnal (active at night) and crepuscular (active at twilight) species, benefit from broad-spectrum lighting". A 2026 evidence review on LafeberVet reports that the leopard gecko, the standard example of a lizard "that does not need UVB", uses it for vitamin D3 synthesis.
How much UVB a species needs was worked out by measuring what wild animals choose. A 2010 study recorded the ultraviolet index that 15 reptile species voluntarily exposed themselves to in the field, and a 2016 open-access paper by Baines and colleagues, working with British zoos, extended it to 254 species and sorted them into four zones. The paper's advice is to build a gradient in the enclosure "from zero (full shade) to the maximum indicated for that zone", so the animal chooses its own dose the way it would outdoors.
| Zone | What the animal does in the wild | Usual exposure, UV index | Highest recorded, UV index |
|---|---|---|---|
| 1 | Crepuscular or shade dweller, thermal conformer | 0 to 0.7 | 0.6 to 1.4 |
| 2 | Partial sun or occasional basker, thermoregulator | 0.7 to 1.0 | 1.1 to 3.0 |
| 3 | Open or partial sun basker, thermoregulator | 1.0 to 2.6 | 2.9 to 7.4 |
| 4 | Mid-day sun basker, thermoregulator | 2.6 to 3.5 | 4.5 to 9.5 |
The zones the paper assigns to the twelve animals on the setup tool: bearded dragon 3 to 4, leopard gecko 1, crested gecko 1, the Australian blue-tongued skink 2 to 3, ball python 2, corn snake 1 to 2, the San Francisco garter snake 2, red-eared slider 3 to 4, Hermann's tortoise 3, Russian tortoise 3, sulcata 3 to 4. Kingsnakes are not in the paper; their milksnake relatives are zone 1. The RSPCA's own targets line up with this: a UV index of 3.0 to 5.0 at the basking spot for a bearded dragon and 0.7 for a leopard gecko.
Dusting is coating the food in a calcium powder just before it is eaten. For insects, RSPCA and Merck describe the same trick: put the insects in a plastic bag, add the powder, and shake gently; for greens, sprinkle it over the plate. Merck says the dusting should happen "immediately before offering them", and RSPCA says uneaten dusted insects come out after 20 to 30 minutes, because the coating falls off and the insect starts eating the reptile's food. The powder should be calcium with little or no phosphorus, and LafeberVet says the supplement itself should have a calcium-to-phosphorus ratio of at least 2 to 1.
How often is where the sources part company, and the safe reading is that growing animals need it far more often than adults. LafeberVet: every meal for hatchling bearded dragons and once or twice a week for an adult, with a separate vitamin and mineral supplement once a week. VCA: daily plain calcium for a young dragon and calcium with D3 two or three times a week for an adult. Washington State: four or five times a week for a juvenile and once or twice for an adult. RSPCA's supplements page: a high-calcium supplement "at every feed" for insect, plant and mixed feeders, with a multivitamin two or three times a week and never daily, because "daily usage of combined multivitamin-multimineral supplements is not required and could result in vitamin overdose". Vitamin D3 in the powder is the other judgment call: RSPCA says with a well set-up UV lamp "you won't need to provide high dietary D3 levels", Merck says dietary vitamin D is only needed when the animal cannot make its own, and LafeberVet's schedules put the D3 in the weekly vitamin rather than the calcium. Follow the label amount; RSPCA warns that vitamins and minerals "can be over-provided".
A cricket is only as nutritious as its last meal, so gut-loading is feeding the insects a calcium-rich diet before they become dinner, "to fill their gastrointestinal tract with nutrients" in RSPCA's words. Commercial gut-loading diets are made for it and are high in calcium; fresh safe vegetables do part of the job and keep the insects alive. The timing is another disagreement: VCA says within 12 hours of feeding, RSPCA's species pages say 24 to 48 hours, its gut-loading page 24 to 72, Merck says 72 hours on a feed containing at least 8 to 10 percent calcium, and LafeberVet says several days. The reading that satisfies all of them is to start the insects on a high-calcium diet one to three days before feeding and keep them on it until they are fed. Gut-loading does not replace dusting; the vets do both.
The species-by-species numbers, including each animal's zone, lamp distance and dusting schedule, are on the reptile setup tool.