Via The Daily Mail in the UK:
The newspaper reports that last Friday 26 year-old Prince Harry was observed inhaling from "several balloons of nitrous oxide provided at £1.50 a time at a charity fancy-dress party in Dulwich, South London."
The article quotes someone at the party remarking that Harry "loves laughing gas. It is only mild and gives you a two-minute high.But if you have had alcohol or other drugs, it can space you out for much longer. Some people pass out after inhaling it."
it also defines nitrous oxide as the propellant in aerosols and a mild anaesthetic which has also become a recreational drug of choice for the middle classes. "It is known as 'hippy crack' for the euphoria it induces."
The article points out that at least two deaths have been attributed to it in the last few years. In 2007, a a 23 year-old man was found dead at his home next to a large cylinder of the gas and in 2009, comedian Martin Cassidy died last year after accidentally overdosing.