A pill could help people cure themselves of a fear of heights, a study suggests.
Scientists have discovered that giving people a tablet of the stress hormone cortisol can help reduce their phobia.
The hormone, which is part of the body's "fight or flight" reaction to danger, appears to open the brain up to being reprogrammed and to permanently remove anxieties.
Tests on 40 patients with acrophobia – a fear of high places and edges – found those given cortisol in combination with behavioural therapy dramatically reduced their aversion.
The researchers said their findings, published in Proceedings of the National Academy of Sciences, could lead to the development of effective treatments for a host of anxiety disorders.
Half the participants were given the drug and the others a placebo an hour before being subjected to a virtual-reality outdoor elevator ride.
Their fear was measured three to five days and one month after the last exposure session through an established acrophobia questionnaire and by sensors that picked up their sweat known as skin conductance examinations.
Compared with those given the dummy pill participants who took cortisol suffered significantly less anxiety and a smaller increase in skin conductance during follow-up.
The effect also lasted a lot longer and still apparent a month later.
Dr Dominique De Quervain, a neuroscientist at Basel University in Switzerland, and colleagues said this suggests cortisol can enhance "exposure therapy" to reduce fear of heights.
This involves repeated, controlled, exposures to fearful situations that gradually dampens their fright.
The researchers said: "Adding cortisol to exposure therapy resulted in a significantly greater reduction in fear of heights as measured with the acrophobia questionnaire both at post-treatment and at follow-up, compared with placebo.
"Furthermore, subjects receiving cortisol showed a significantly greater reduction in acute anxiety during virtual exposure to a phobic situation at post-treatment and a significantly smaller exposure-induced increase in skin conductance level at follow-up.
"The present findings indicate that the administration of cortisol can enhance extinction-based psychotherapy."
They said the results add further evidence to research showing fear can be controlled by drugs and such studies may contribute "to the development of novel therapeutic strategies to treat anxiety disorders."
Acrophobia triggers a sense of panic when at height and is not the same as vertigo that causes a feeling of spinning and dizziness.
Common reactions include descending immediately, crawling on all fours and kneeling or otherwise lowering the body.
Cortisol is a hormone made by the adrenal glands. Some disorders such as arthritis, skin disorders and asthma can be treated with synthesised cortisol, called cortisone or corticosteroids.