Photosensitization is a condition in which a pet’s skin overreacts to light because a light-sensitive substance has built up in it. It can look like sunburn, but it is a separate problem with a different cause [1]. If your dog or cat seems uncomfortable in the sun and the pale parts of the face turn red and swollen, bring your pet indoors or into full shade and call your vet.
Sun damage itself, the kind that builds up over years on pale noses and ear tips, is covered in our posts on a dry, crusty nose in dogs, lumps and bumps on cats and ear problems in cats. This post is about something different: a substance in the body that makes the skin react to light.
A note on the evidence: most research on photosensitization comes from grazing animals such as cattle, sheep and horses, where plants are the usual trigger [1]. Reports in dogs and cats are few. Where this post describes how the condition works in general, it draws on that wider veterinary research; where it talks about dogs and cats, it relies on published case reports.
What happens in the skin
A photosensitizing substance absorbs light energy and passes it on, which produces reactive forms of oxygen inside skin cells. These damage the cells and set off inflammation [1]. Sunburn is mainly caused by too much ultraviolet light; photosensitization can also be triggered by visible light, so it covers a wider range of light than ordinary sunburn [1].
Dark skin pigment gives some protection, which is why pale, thinly haired or hairless skin is usually hit hardest [1].
What causes it
Vets usually sort photosensitization by where the light-sensitive substance comes from [1]:
- Primary: a light-sensitive substance is eaten or touches the skin and reaches the skin directly [1].
- Faulty pigment production: the body itself makes too much of a group of pigments called porphyrins, which it normally turns into heme, the part of blood that carries oxygen. Some researchers group this with the primary type [1][2].
- Liver related (secondary): the liver normally clears certain light-sensitive breakdown products from the body. When the liver is damaged, these build up and reach the skin [1].
Because the liver can be involved, your vet may want to check liver health in a pet with suspected photosensitization [1].
Medicines
Some medicines, including some antibiotics, can make skin react to light [1]. A case series described suspected reactions of this kind to antibiotics in three dogs [3]. If your pet’s skin reacts to sunlight while on a medicine, tell your vet rather than stopping the medicine on your own.
Inherited pigment disorders: two case reports
In people, inherited porphyrias that affect the skin can cause pain, redness, swelling or blisters on skin exposed to sunlight [2]. Rare cases have been described in pets:
- A dog: a young Clumber Spaniel had poor growth, repeated skin inflammation on the head and a worsening liver problem. Testing found a rare inherited pigment disorder (congenital erythropoietic protoporphyria), and the authors thought the skin disease was caused by light sensitivity from it. Care included keeping the dog out of the sun along with liver treatment. At 4 years of age the dog was stable, though sun-related skin disease was likely still present [4].
- A cat: an adult domestic shorthair cat with a related inherited disorder (congenital erythropoietic porphyria) had dark-red urine and brownish teeth that glowed red under ultraviolet light [5].
These are single case reports. They show what these rare disorders looked like in one dog and one cat, not what to expect in every pet.
Signs
- Restlessness and discomfort in sunlight, with the animal looking for shade [1].
- Redness of the pale or sparsely haired parts of the head, such as the muzzle, ears and the skin around the eyes [1].
- Swelling of the eyelids and the skin around the eyes [1].
- As it goes on, oozing that dries into thick crusts, and patches of skin that die and peel away [1].
- Damaged skin can become infected [1].
If the liver is the cause, the animal may also seem unwell and dull and show yellowing of the skin, gums or eyes (jaundice). Jaundice is not expected when the cause is primary [1].
Timing varies. Signs can start within minutes after direct skin contact, within hours when the substance comes from the bloodstream, or days later when liver damage is behind it [1].
How it is diagnosed
Your vet will examine the skin, ask about medicines and about anything your pet may have eaten or been exposed to, including poisons, and look for an underlying illness. Blood tests help tell the types apart: in primary photosensitization the liver results are usually normal, while in the liver-related type liver enzymes and bilirubin are usually raised [1]. A skin or liver sample may be needed in some cases [1][4]. For the inherited pigment disorders, porphyrin levels can be measured in blood, urine and feces [4].
Treatment and outlook
The first step is to limit sunlight exposure, with full shade or, better, keeping your pet indoors until your vet says otherwise [1][4]. Your vet will look for the cause, treat any liver problem and decide how to care for the damaged skin and any infection. Damaged skin can attract flies, so keep your pet away from them while it heals; our post on maggots and warbles in dogs and cats explains why.
Severe skin damage can make an animal seriously ill and can even be fatal [1]. How a pet does depends on the cause, how badly the skin and liver are affected and how well the skin heals. Your vet can explain what this means for your pet.
What you can do right now
Move your pet out of direct sunlight, into full shade or, better, indoors, and call your vet. Your vet will decide on any treatment for the skin.
When to see your vet
See your vet if your dog or cat:
- seems restless or uncomfortable in sunlight and keeps rubbing or scratching the ears, eyelids, muzzle or nose
- has redness and swelling on pale or hairless skin after being in the sun [1]
- has oozing, crusts or areas of dead-looking skin [1]
- has yellowing of the skin, gums or eyes along with skin changes [1]
- has skin changes and may have been exposed to rat poison, toxic plants or other poisonous chemicals
- has skin that reacts to sunlight while taking a medicine [1][3]
FAQ
Is photosensitization the same as sunburn?
No. The two can look alike, but photosensitization is caused by a light-sensitive substance in the skin, while sunburn comes mainly from too much ultraviolet light [1].
Can a dark-coated dog or cat be affected?
Dark skin pigment gives some protection, so pale, thinly haired or hairless areas are usually affected first and worst [1]. That protection is partial, so any pet whose skin reacts to sunlight should see a vet.
Why would my vet check my pet’s liver for a skin problem?
Because a damaged liver can fail to clear light-sensitive substances, which then build up in the skin. Blood tests help show whether the liver is involved [1].
Will the skin heal?
It depends on the cause and on how badly the skin and any liver problem are affected [1]. Your vet can tell you what to expect for your pet.
Can a medicine cause it?
Yes, some medicines, including some antibiotics, can make the skin react to light, and a case series describes suspected reactions in three dogs [1][3]. Do not stop a medicine on your own; talk to your vet.
The bottom line
Photosensitization makes the skin overreact to light because a light-sensitive substance has built up in it; it can look like sunburn but has a different cause [1]. Pale and hairless skin on the head is usually affected first, with redness and swelling that can progress to oozing, crusts and dead skin [1]. Because liver damage is one cause, your vet may test the liver [1]. Keeping your pet fully shaded or, better, indoors is the first step while your vet finds the cause [1].
This article is for general information and does not replace a visit to your veterinarian. If you are worried about your pet’s skin, contact your vet.
Sources
- Péli ER, Cserhalmi D. Primary and Hepatogenous Photosensitization in Livestock: A Review of Plant-Derived Phototoxins and Veterinary Implications. Vet Sci. 2026;13(3):214. Open access, CC BY 4.0. https://pmc.ncbi.nlm.nih.gov/articles/PMC13030745/
- National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK), National Institutes of Health. Porphyria. Last reviewed July 2020. https://www.niddk.nih.gov/health-information/liver-disease/porphyria
- Lewis DT, Craft WF, Rossi MA, Pieper JB. Putative antimicrobial-associated phototoxicity in three dogs. Vet Dermatol. 2023;34(4):355-358. https://pubmed.ncbi.nlm.nih.gov/36794534/
- Kunz BC, Center SA, Randolph JF, et al. Congenital erythropoietic protoporphyria and protoporphyric hepatopathy in a dog. J Am Vet Med Assoc. 2020;257(11):1148-1156. https://pubmed.ncbi.nlm.nih.gov/33226294/
- Clavero S, Bishop DF, Giger U, et al. Feline congenital erythropoietic porphyria: two homozygous UROS missense mutations cause the enzyme deficiency and porphyrin accumulation. Mol Med. 2010;16(9-10):381-388. https://pubmed.ncbi.nlm.nih.gov/20485863/