In rare cases, kissing can trigger an allergic reaction
The strange case came to light when an Italian woman went to hospital with swollen lips.
Medics found that a passionate clinch with her husband half an hour earlier was to blame.
She had reacted to a drug he was taking for an infection.
Foods eaten by their lovers turned out to be the trigger factor.
Some people are so allergic to the likes of peanuts or apples that they can react even to traces left on someone's lips.
But this is thought to be the first time that someone has suffered an allergy to medicine taken by a partner.
The case was reported in medical journal The Lancet by experts at the allergy clinic of A Cardarelli Hospital in Naples, Italy.
They proved their theory by giving the 45-year-old's husband a dose of the same medicine - an antibiotic - and getting him to kiss his wife.
After about 20 minutes, the woman came out in a rash.
Food sensitivities
Allergy specialists in the UK say the finding, while extremely rare, is not that surprising.
Dr John Mansfield, medical director of the Burghwood allergy clinic in Banstead, Surrey, said it is well known that babies are extremely sensitive to food in their mothers' milk.
He has also treated adult allergy patients who react to someone chewing a peanut in the same room.
"The biological system, including human beings, can be mega sensitive," he told BBC News Online.
"If you can pick up a peanut sensitivity from walking past someone chewing a peanut you can certainly pick up a sensitivity from kissing."