Using a Blacklight to Discover Dog Pee Accidents

Author: Marissa Busch

Published: February 21, 2019

Updated: February 28, 2020

This page may contain affiliate links. We earn a commission for qualifying purchases – at no cost to you. Our mission is to help save dogs' and cats’ lives through our educational content. To help us create more veterinarian- and trainer-approved content, please consider buying one of our web-books for yourself or as a gift.

Dog pee glows under blacklight
Despite your best efforts, sometimes your dog’s potty training may not be going exactly according to plan. If your dog isn’t always making it outside to potty, pee stains and odors can be lurking in your carpets. You may not even know that your dog is secretly going potty in your home until you smell a hint of pee.

How do you know if your dog has been having accidents indoors?

Use a blacklight!

If there are no smells or your dog isn’t repeatedly marking one area, it doesn’t mean you’re in the clear. Rather than putting your nose to the floor trying to discover where that smell is coming from, you can find dried urine stains on carpet and furniture with a blacklight. Unleash your inner scientist and scan the light on your floors to find the stains. 

Pro-Tip: A blacklight works best the darker the room is. Try to do your searching at night, unless you’ve got good blackout curtains or other means of really darkening a room during the day.

Wavelengths in a blacklight cause phosphorous and proteins in the urine to glow, making it easier to spot old stains. Pet stains that weren't cleaned properly with an enzyme-based cleaner will show up under UV light as well.

This UV flashlight is less expensive, however, the wavelength from this UV Blacklight Torch (365–370nm) is more effective — but it's a pricey gadget. (FYI: The greater the wavelength, the better visibility for fluorescing proteins in urine.) Both of these lights will work, but the less expensive light will typically only work in a very dark room.

Where to search

You can’t always see pee stains, especially on dark carpet, and smells from pee stains left on the carpet can cause your dog to repeatedly mark that spot. If you’re curious if your dog has been having accidents, here are some places you can start your search:

Pro-Tip: Check the walls and furniture for splatters next to any stains you find, like shown in the photo below (especially true if you have a male dog).
  • Where you've caught your dog having accidents before
  • Behind curtains
  • Near entry doors
  • Dark corners
  • On and around their bed
  • Under their crate
  • Next to indoor plants

Couch dog pee splatter under blacklight

What now?

Pro-Tip: When moving into a new home, it can be helpful to use a blacklight to see if there are any messes that the previous tenant or homeowner didn’t clean properly to prevent your dog from having accidents in the same places.

Once you’ve found the urine stains, it’s time to clean them up with an enzyme-based cleaner that will neutralize the odors and remove the stain. Use our tips for cleaning up potty accidents to make sure you only have to do this once.

Don’t have a blacklight handy, but want to check your floors? Check out this “hack” for turning your iPhone flashlight into a makeshift blacklight:

If you’ve found and cleaned the urine spots on your floors and your dog is still having accidents inside, your dog may have a medical issue (e.g., diabetes, Cushing’s disease, kidney dysfunction, or a UTI) that could be making it more difficult to “be a good dog.” If you have concerns, always consult your veterinarian. And if they ask you to bring a pee sample by, check out this article on how (and why) to collect a dog’s pee sample.

potty accidents can be a sign of canine separation anxiety

Not all that glows is dog pee!

When exposed to ultraviolet beams of a black light, lots of things will glow white, green, blue, orange, or even red. While dog pee glows under this type of light, so will liquids and other liquid-like substances, such as some laundry detergents, milk, honey, canola or olive oil, ketchup, tonic water, and antifreeze. Blacklight is also used to detect molds and fungi, including some of the fungi species that cause ringworm.

Want to experiment with a black light on some fun things in your home? Try lighting these up: cotton balls, brown spots on banana peels, some flowers, B-vitamins, and newer US currency over $5 (they have fluorescent bands to prove they’re not counterfeit). 

More resources:


*Learn about Amazon links and Preventive Vet recommended products  

Must-have digital books for dog and cat owners