Fearful memories can be erased using a drug commonly prescribed for high blood pressure, research has shown.
The discovery could lead to new ways of treating people suffering from the emotional after effects of traumatic experiences such as terrorist attacks or natural disasters.
But one British expert questioned the ethics of meddling with a person's mind in this way.
Previous research on animals had shown fear memories are susceptible to being altered at the time they are recalled.
At this crucial stage the memories are "reconsolidated" in the brain. Studies suggested beta-blockers, a family of drugs normally used to treat high blood pressure, may interfere with the reconsolidation process.
Now a trial involving human volunteers has turned theory into fact.
A team of Dutch researchers artificially created a fearful memory by associating pictures of spiders with a mild electric shock delivered to the wrists of the 60 participants.
When the volunteers were shown the spider pictures 24 hours later their "startle" response - a measure of fear - was assessed by testing eyeblink reactions.
Administering the beta-blocker drug propranolol before reactivation of the fearful memory led to a marked reduction in the startle response.
After taking the drug, volunteers were much less disturbed by the spider pictures.
Strikingly, the effect appeared to be permanent. The spider fear seen in the initial experiment did not return to treated participants.
The findings, reported today in the journal Nature Neuroscience, raise the possibility of a new approach to tackling emotional problems and post-traumatic stress disorder.
Study leader Professor Merel Kindt and colleagues from the University of Amsterdam wrote: "Millions of people suffer from emotional disorders and the relapse of fear, even after successful treatment.
"Our findings may have important implications for the understanding and treatment of persistent and self-perpetuating memories in individuals suffering from emotional disorders."
However Dr Daniel Sokol, lecturer in Medical Ethics at St George's, University of London, sounded a word of caution.
He said: "Removing bad memories is not like removing a wart or a mole. It will change our personal identity since who we are is linked to our memories. It may perhaps be beneficial in some cases, but before eradicating memories, we must reflect on the knock-on effects that this will have on individuals, society and our sense of humanity."
John Harris, Professor of Bioethics at the University of Manchester, said: "It is obviously up to the individual whether or not she wishes to risk the possible effects, including psychological discontinuity, of erasing unpleasant memories.
"An interesting complexity is the possibility that victims, say of violence, might wish to erase the painful memory and with it their ability to give evidence against assailants.
"Similarly, criminals and witnesses to crime may, under the guise of erasing a painful memory (perhaps of another sort), render themselves unable (with a good excuse for being unable) to give evidence."
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