Jelly pieces, laced with a homeopathic remedy containing arsenic and sulphur, can prevent the animals from contracting mange.
Attempts in the Scottish Parliament to prevent the hunting of foxes with dogs have been fiercely resisted by the pro-hunting lobby.
But wildlife carers in the city and suburbs have been worried about the welfare of foxes, and about the animals spreading the disease to their pets.
Martin Hemmington of the National Fox Welfare Society said: "Generally its unlikely foxes will spread disease. But sarcoptic (canine) mange is one of the most serious diseases a fox may suffer from.
"But we have a new homeopathic remedy - a mixture of arsenicum and sulphur - for treating mange. This can easily be given to foxes on a jam sandwich."
Sarcoptic mange is caused by a tiny mite which burrows into the skin and leaves eggs behind, which cause severe skin irritation.
A fox suffering from mange will often act abnormally.
It will often walk around aimlessly during daylight hours constantly biting and scratching at the irritation, and it will become severely dehydrated.
Light snack
Left untreated it can lead to the death of the infected fox.
Mr Hemmington said jam sandwiches left out for healthy foxes are the ideal light snack to attract them into a garden so they can be watched at night.
Suburban and city animal lovers often make the mistake of leaving out piles of food for a hungry fox.
"By putting out too much food a large population of adult foxes may build up and in such a dense community mange may be more easily spread than in the countryside. "
He said his society had spent £18,000 in distributing the homeopathic paste to fox-friendly people all over the UK - particularly in Glasgow and Edinburgh.
Research by the Scottish Agricultural Science Agency revealed the massive number of red foxes living in the two cities.
Urban foxes
The agency's Dr Gill Hartley said the numbers of fox family groups in Edinburgh totalled 633, and Glasgow had 215 groups.
She said: "We intend to carry out research into the numbers of urban foxes in towns such as Dundee Aberdeen and Inverness."
The Scottish Society for the Prevention of Cruelty to Animals (SSPCA) has offered advice on discouraging garden foxes.
It urged householders to avoid leaving bags full of household rubbish where they can be ripped open; do not leave scrap food on a compost heap and spray an animal repellent round the garden.