A cannabis folk remedy has been resurrected by scientists who found that active ingredients in the drug reduce allergic reactions.
The research, conducted on mice, points the way towards new cannabis-based treatments for irritated skin.
Extracts from the hemp plant were traditionally used to treat inflammation and could be bought from chemists in the early part of the 20th century.
But fears about the intoxicating effects of tetrahydrocannabinol (THC), the chemical that causes the cannabis high, led to a ban on sales in the 1930s.
The new research suggests that the herbalists who used cannabis ointments to treat eczema knew what they were doing.
Scientists now believe that cannabis skin lotion, in a safe form too diluted to affect the brain, could make a comeback. The team from the University of Bonn in Germany stumbled on the anti-inflammatory effect of THC while conducting a brain study on mice. The animals were genetically engineered so they could not respond to cannabinoids, either THC or its natural equivalents generated in the brain.
Unexpectedly, the skin around ear clips placed on the mice to identify them became red and sore.
The scientists realised what this meant - that cannabinoids act like a brake, preventing the immune system from running out of control and triggering inflammation.
For 20 years scientists have known that the brain produces cannabinoids, but it has not been clear why.
They appear to have psychological effects, and influence bone growth. Another possible explanation now seems to be that they help regulate the immune system.
The German scientists confirmed their suspicions by dabbing THC ointment on the skin of mice exposed to allergens. Professor Thomas Tuting, a member of the team, said: "If we dabbed THC solution on to the animals' skin shortly before and after applying the allergen, a lot less swelling occurred than normal.
"The THC attaches itself to the cannabinoid receptors and activates them. In this way, the active substance reduces the allergic reaction."
He said the amount of THC needed to treat skin allergies would be far too small to produce intoxicating effects.
Another option was to develop drugs which prevented the breakdown of natural cannabinoids in the brain.