The 18-year-old is believed to have used nitrous oxide, which can cause loss of blood pressure, fainting and heart attacks.
An 18-year-old has died after he is thought to have taken laughing gas at a party.
Emergency services called to the scene in Bexleyheath, southeast London, found the young victim had gone into cardiac arrest.
The teenager was taken to hospital after collapsing in the street at around 11.20pm on Saturday but died two hours later.
He is believed to have been drinking alcohol as well as using nitrous oxide, according to police.
The gas, usually inhaled through balloons, is nicknamed laughing gas as it can make people feel relaxed and giggly.
But the legal high can also cause loss of blood pressure, fainting and even heart attacks.
The Metropolitan Police said: “Next of kin have been notified. A post-mortem will be scheduled in due course.
“At this stage the death is being treated as unexplained pending the findings of the post-mortem.”
Research has found around 8% of 16-to-24-year-olds in England and Wales admit to having tried balloons containing the gas.
They are reported to be widely available in shops that sell legal highs.