Updated November 2026 · By the Dragon's Diet Team · Lighting information checked against the manufacturer instructions and veterinary sources listed at the end of this article.
Short answer: Replace a reptile UVB bulb on the schedule in its instructions, even if it still lights up. Most current Zoo Med and Arcadia T5, T8 and mercury vapor bulbs are rated for about 12 months. If you don't know the brand, VCA Animal Hospitals advises every six months. Only a UV meter tells you for sure.
A UVB bulb can look perfectly fine and still be failing your reptile. Bearded dragons and other desert reptiles use UVB to make vitamin D3, which they need to absorb calcium. When the UVB fades, the bulb keeps glowing, so you won't see the problem until your dragon's health shows it. Here's how long each type of bulb lasts and how to stay ahead of it.
UVB bulb replacement schedule by bulb type
| Bulb type | Typical replacement interval | Source |
|---|---|---|
| Linear T5 HO fluorescent tube | 12 months | Zoo Med ReptiSun T5 HO instructions (at 10–12 hours a day); Arcadia T5 D3+ Dragon and D3+ Desert product pages |
| Linear T8 fluorescent tube | 12 months for Arcadia D3 and D3+ T8 tubes | Arcadia D3 Forest lamp page |
| Compact (coil) fluorescent | 12 months for current Zoo Med ReptiSun 10.0 compacts; older packaging said 6 months | Zoo Med ReptiSun 10.0 compact instructions |
| Self-ballasted mercury vapor | 12 months | Zoo Med PowerSun UV product page |
| Unknown brand or no instructions | Every 6 months, or as the manufacturer directs | VCA Animal Hospitals |
| Basking or heat bulb (no UVB) | When it burns out; it doesn't provide UVB | — |
Always check the instructions in your box first. Ratings change between product versions, and the Royal Veterinary College notes that some lamps need replacing every 3–6 months while others last 6–12 months. Follow the maker's schedule unless you measure your bulb with a UV meter.
Why UVB fades even when the bulb still lights up
A UVB bulb makes two kinds of light: the visible light you can see and the ultraviolet light you can't. The UVB output drops over time while the visible light stays bright. VCA Animal Hospitals says to replace UVB bulbs on schedule even if they still produce visible light, and that's the key point: a working light isn't a working UVB bulb.
That matters because UVB is how your bearded dragon makes vitamin D3. Without enough of it, a dragon can't absorb calcium well, even with a good diet, and over time that can lead to metabolic bone disease. Our metabolic bone disease guide explains the risks, and our calcium and vitamin D3 guide covers how UVB and supplements work together.
Replacement schedule details by bulb type
Linear T5 HO tubes
Short answer: Replace a quality T5 HO UVB tube about once a year. Zoo Med rates its ReptiSun T5 HO for 12 months at 10–12 hours a day, and Arcadia says to change its T5 D3+ Dragon and Desert tubes once a year.
T5 HO tubes are the setup we recommend for bearded dragons. They spread UVB along most of the enclosure, so your dragon can move in and out of it. Our bearded dragon tank setup guide covers which strength to choose, where to mount it and the UV index to aim for at the basking spot.
Linear T8 tubes
T8 tubes put out less UVB than T5 HO tubes, so they have to sit closer to your reptile. Arcadia says its D3 and D3+ T8 tubes are effective for one whole year. Older or bargain T8 tubes may not last as long, so follow the packaging or use a meter.
Compact (coil) bulbs
Compact UVB bulbs screw into a standard dome fixture. Zoo Med's current instructions for the ReptiSun 10.0 compacts say to replace them every 12 months, while older versions of the same bulb said every 6 months. That's a good example of why you should go by the instructions that came with your bulb. Compacts light a narrow, uneven patch, so we don't recommend them as the main UVB for a bearded dragon.
Mercury vapor bulbs
Self-ballasted mercury vapor bulbs give off heat and UVB from one bulb. Zoo Med's PowerSun UV is rated for 12 months, so replace it once a year. They run hot, so keep a safe distance from your reptile and follow the instructions for fixture and wattage.
Basking and heat bulbs
A basking bulb like our Fluker's 100-watt basking bulb provides heat and light, not UVB. Your dragon needs both a basking bulb and a separate UVB source. Replace heat bulbs when they burn out, and keep a spare so a burned-out bulb doesn't leave your dragon cold overnight before you can get a new one.
How to keep track of your UVB bulb
- Write down the start date. Zoo Med's instructions say to record the date you first use the bulb. A strip of tape on the fixture or a phone reminder works.
- Set a reminder for the replacement date. Zoo Med also offers a free UVB reminder tool on its website.
- Run it on a timer. Zoo Med rates its T5 HO tubes at 10–12 hours a day, so a timer keeps your day length steady and your bulb on schedule.
- Measure if you can. A UV meter that reads the UV index lets you check the actual output at the basking spot instead of guessing. The Royal Veterinary College says to follow the manufacturer's schedule unless you have one.
- Buy the replacement before you need it. Swap it in on the date, not when you remember.
Mistakes that weaken UVB before replacement time
- Glass or plastic in the way. UVB doesn't pass through ordinary glass or plastic, so a lamp sitting on a glass lid does little. Mount it inside the enclosure or over a mesh top.
- Mesh tops. Zoo Med notes that fine mesh can block up to 50% of UVB, which affects how close the bulb needs to be.
- Wrong distance. VCA advises the bulb be 6 to 18 inches from your dragon. Follow your bulb's own distance guidance.
- Trusting "full spectrum" labels. VCA notes that a "full spectrum" label doesn't guarantee a bulb produces UVB.
- No measurement after a new bulb. Arcadia measures its stated output after the first 100 hours of use, and Zoo Med notes a burn-in period for its compacts, so a brand-new bulb may run stronger in its first couple of weeks.
Signs your bearded dragon may not be getting enough UVB
By the time your dragon shows signs, the problem has been building for a while, which is why replacing on schedule matters. Watch for:
- Lethargy or less interest in food
- Trouble walking, weakness or trembling
- Swollen limbs or a soft, rubbery jaw
If you notice any of these, replace an old bulb and book a visit with a reptile vet.
UVB, calcium and good feeders work together
UVB lets your dragon use the calcium it eats, so diet matters just as much. Dust insects with calcium and feed gut-loaded insects. Our live Dubia roaches are gut-loaded before they ship and come in Small (3/8"), Medium (1/2") and Large (3/4"). Every order ships USPS Priority with a 100% live arrival guarantee when you follow our live arrival policy (send a photo within 3 hours of delivery). New customers can use code NEWCUSTOMER on their first order. Browse our reptile heating and lighting supplies too.
When to Replace a UVB Bulb FAQ
How often should you replace a UVB bulb?
Follow the schedule in your bulb's instructions. Most current quality bulbs from Zoo Med and Arcadia, including T5 HO tubes, T8 tubes and mercury vapor bulbs, are rated for about 12 months. If you don't know the brand or have no instructions, VCA Animal Hospitals recommends replacing it every six months.
Do UVB bulbs need to be replaced if they still work?
Yes. UVB output fades over time while the bulb keeps producing visible light, so a bulb that still glows may no longer give your reptile enough UVB. VCA Animal Hospitals says to replace UVB bulbs on schedule even if they still produce visible light. A UV meter can confirm the actual output.
How long does a T5 UVB bulb last?
A quality T5 HO UVB tube is typically rated for about 12 months. Zoo Med rates its ReptiSun T5 HO for 12 months at 10 to 12 hours a day, and Arcadia says to change its T5 D3+ Dragon and Desert tubes once a year. Record the start date so you know when it's due.
Is a 6-month or 12-month UVB replacement schedule right?
Both appear in good sources. Many current T5, T8 and mercury vapor bulbs are rated for 12 months, while VCA Animal Hospitals advises every six months unless the manufacturer says otherwise. Go by your bulb's instructions, use six months when you're unsure, and use a UV meter if you want to know for certain.
Does a basking bulb provide UVB?
No. A standard basking or heat bulb gives heat and visible light but not UVB. Bearded dragons need a basking bulb for heat and a separate UVB bulb, ideally a linear T5 HO tube, for vitamin D3. Replace heat bulbs when they burn out and UVB bulbs on their own schedule.
What happens if I don't replace my bearded dragon's UVB bulb?
As UVB fades, your dragon makes less vitamin D3 and can't absorb calcium as well, even with supplements. Over months, that can lead to metabolic bone disease, with signs like weakness, trembling, swollen limbs or a soft jaw. Replace the bulb on schedule and see a reptile vet if you notice symptoms.
Sources
- Zoo Med, "ReptiSun T5 HO 5.0 and 10.0 instructions"
- Zoo Med, "ReptiSun 10.0 Mini Compact instructions"
- Zoo Med, "PowerSun UV"
- Arcadia Reptile, "D3+ Dragon 14% UVB T5 lamp" and "D3 Forest lamp" (T5 and T8)
- VCA Animal Hospitals, "Housing Bearded Dragons"
- VCA Animal Hospitals, "Lighting Requirements for Reptiles"
- Royal Veterinary College, Beaumont Sainsbury Animal Hospital, UVB lighting factsheet (2022)
This guide is general husbandry information, not veterinary advice. Ask a reptile veterinarian about your pet's lighting and health, especially for a young, new, or unwell reptile.
About the author
Written by the Dragon's Diet Team. We've kept and cared for reptiles for 25 years, and we raise and ship live feeder insects from Taylor Mill, Kentucky. Our care guides draw on that hands-on experience and on veterinary and manufacturer sources like VCA Animal Hospitals, Zoo Med and Arcadia.
