Laughing gas can help you forget unfunny distressing memories, a study has shown.
Scientists found that volunteers who watched violent film clips were less bothered by upsetting recollections of what they had seen after inhaling nitrous oxide.
For the research, 50 healthy adults were made to sit through two graphic scenes from a film once described as "so violent and cruel that most people will find it unwatchable".
After the seeing the clips participants were either asked to breath ordinary air or a mixture of laughing gas and oxygen for 30 minutes.
In the week that followed, they kept a diary to record the number of intrusive memories they experienced related to the movie scenes.
Lead scientist Dr Ravi Das, from University College London (UCL), said: "The day after they saw the film, the number of intrusions experienced by the group who received nitrous oxide fell by over a half.
"By contrast, the decline in intrusions was much slower in the group who received air, where there was not a significant drop in intrusions until the fourth day.
"We think that this is because nitrous oxide disrupts a process that helps permanent memories to form."
Nitrous oxide is thought to block a brain process during sleep that files away "important" memories - such as those that produce a strong emotional response - for long-term storage.
The research, published in the journal Psychological Medicine, also found that people who experienced more "dissociation" - a dream-like, detached feeling - after watching the film clips were more vulnerable to intrusive memories.
Dr Sunjeev Kamboj, another member of the UCL team, said: "Nitrous oxide is routinely used as a painkiller by paramedics and in A&E departments because it is safe and easy to administer.
"Many people who end up in an ambulance will have undergone some form of psychological trauma, and our study suggests that the nitrous oxide is likely to be having some effect on how their brain processes it.
"However, whether it helps to prevent symptoms of PTSD (post-traumatic stress disorder) or makes them more likely may depend on how dissociated patients feel before they receive it.
"Further research is now needed to determine whether dissociation similarly affects the response of trauma victims who receive nitrous oxide or other painkillers such as ketamine."