A teenager, believed to have taken laughing gas at a party, is in a critical condition.
The 18-year-old was rushed to hospital after police and the ambulance service attended the scene in Bexley, south-east London.
Police said the teen is believed to have been drinking alcohol as well as ingesting legal high nitrous oxide, also known as laughing gas.
He was in cardiac arrest when officers arrived at the scene in Wolvercote Road at around 11.18pm on Saturday and is now in a critical condition in hospital.
Research published earlier this year showed there had been a rise in the recreational use of nitrous oxide, known by various names including "hippie crack".
It is not illegal to be in possession of the substance although it is prohibited from being sold in England and Wales to under-18s if there is a risk they will inhale it.
The gas can make people feel relaxed, euphoric and giggly, leading to the nickname laughing gas. It is usually inhaled through balloons.
Abusing nitrous oxide can lead to oxygen deprivation resulting in loss of blood pressure, fainting and even heart attacks.
The research by respiratory consultant and neonatal paediatrician Dr Paul Seddon found that 7.6% of 16-to-24-year-olds in England and Wales have admitted to having tried balloons, which were described as "widely available" in shops that sell legal highs.
Seventeen people in the UK died between 2006 and 2012 after taking the legal high.