A side-effect of a common eye drug could lead to its use as a baldness treatment.
Lumigan is administered as eye drops to patients with glaucoma, a condition caused by excess fluid in the eye that leads to vision loss.
But it has one marked side-effect -- it can stimulate the growth of eyelashes.
New research suggests the active ingredient in Lumigan, bimatoprost, has the same effect on the scalp.
Tests identified a previously unknown molecular signalling pathway linked to hair growth from follicles.
Preliminary trials are now under way to see whether bimatoprost can reverse hair loss in both men and women.
If successful, it may not be long before the drug is remarketed as a baldness treatment.
Lead scientist Professor Valerie Randall, from the University of Bradford in the UK, said: "Bimatoprost is known to stimulate eyelash growth and is already used clinically for this purpose. We wanted to see whether it would have the same effect on scalp hair, as the two types of follicle are very different.
"Our findings show that bimatoprost does stimulate growth in human scalp hair follicles and therefore could offer a new approach for treating hair loss."
Male hormones -- or androgens -- can both stimulate the growth of hair on the chest and chin, and suppress it on the head.
They are responsible for classic male pattern baldness, marked by a receding hairline and expanding bald patch in the middle of the scalp.
Androgens stimulate signalling pathways linked to baldness by latching on to specific receptor molecules on cells. However, bimatoprost acts on a quite different receptor, unconnected with androgens.
"Even if the androgen receptor is telling the hair follicle to do negative things, the drug is telling it to do positive things," said Prof Randall.
The drug was tested on living scalp tissue obtained from volunteer donors undergoing cosmetic surgery. Follicles treated with bimatoprost grew a third more hair than untreated samples in just nine days.
Scalp follicles were found to contain the same receptors responsive to bimatoprost as eyelash follicles.
"This means that -- so long as the drug can be applied so it can reach the follicle -- it should stimulate hair growth in patients," said Prof Randall.
Results from clinical trials taking place in the US and Germany should be available before the end of the year.
They involve 220 men with male pattern baldness and 172 women with female pattern baldness.
Participants are undergoing six months' treatment either with a solution of bimatoprost, applied to the scalp, or a placebo. A comparison with the baldness treatment minoxidil is also being assessed.
Prof Randall acts as a consultant to Allergan Inc, which manufactures Lumigan.