YouGov recently polled 2,096 British adults on their perceptions of the safest and most dangerous drugs.
They found alcohol was seen as the safest drug, ahead of cannabis, laughing gas and magic mushrooms. Heroin was seen as the most dangerous. Here are those perceptions plotted against data from a 2010 Lancet study on the most harmful drugs in the UK, led by Professor David Nutt. This chart is ranked from what drugs are perceived as the most dangerous from YouGov's polling and reality.
It's not a perfect comparison: the YouGov poll only asked about 11 drugs, and the Nutt study is older and did not look at laughing gas. But it is an interesting snapshot which gives some indication of what the public think and the actual reality.