The occasional lick on the face from a pet dog or cat may seem harmless, but pet owners prescribed 5-fluorouracil have been urged to put a leash on such behaviour.

In a letter published in the British Journal of Dermatology, researchers suggest patients given the topical treatment for actinic keratoses need to be warned about its extreme toxicity to their canine and feline companions.

The adverse effects of 5-fluorouracil — erythema, irritation, photosensitivity and prolonged inflammation — are well known.

But the letter authors, led by dermatologist Dr Bernard Ho from St George’s Hospital in London, say many patients and clinicians alike are unaware of its “detrimental toxic effects” on animals.