A drug used to treat tuberculosis may also help people to overcome a fear of spiders or other phobias.
The drug may help those afraid of spiders
Doctors in the United States believe D-cycloserine could also help people with post-traumatic stress disorder.
Tests on 30 patients terrified of heights has found the drug, when used in combination with therapy, helped them overcome their fears.
While further research is needed, experts told Chemistry & Industry magazine the findings were exciting.
Millions of people around the world suffer from phobias. Those with severe phobias sometimes have a form of psychotherapy called cognitive behaviour therapy to help them overcome their fears.
Studies have suggested that this type of therapy, which generally involves patients confronting their fear, is very effective and that improvements can be seen after just eight sessions.
However, this latest study by Dr Michael Davis and colleagues at Emory University School of Medicine in Atlanta, Georgia, suggests improvements could be achieved more quickly.
The researchers gave D-cycloserine to 20 of the 30 patients who were afraid of heights.
Half received a 50mg dose of the drug and the other half received a much higher 500mg dose. The remaining 10 patients were given a dummy drug.
The patients were then exposed to their fear by placing them in simulated glass elevators that appeared to go up and down.
"As the elevator went higher, they became more and more anxious," said Dr Davis.
"But those taking D-cycloserine learned much more quickly to overcome their fear - in two sessions rather than the usual eight."
The 50mg dose was as effective as the 500mg dose.
Dr Davis said he believed the drug would help people with a wide range of phobias. It could also help people overcome nerves when learning new skills, such as horse-riding.
"It should help you get over whatever it is you are afraid of," he said, "as long as you face up to your fear."
Dr Davis suggested the drug helps to speed up the process of learning and forming memories, helping patients to learn not to be afraid.
He is now planning to carry out another study to see if the drug can help people with a fear of public speaking.
Michael Otto, a professor of psychiatry at Harvard University, hailed the findings.
"These results are very exciting," he said. "They represent a new direction for combining medication and cognitive behavioural approaches to psychotherapy."
He is now planning to test the drug to see if it works with people who suffer from panic attacks.
"Our study will examine the potential for individual doses of D-cycloserine to speed treatment and improve response to exposure-based treatment for panic disorder," he said.