Professor David Nutt of Imperial College London called for a new system of classifying drugs to enable the public to better understand the relative harm of legal and illegal substances.
Alcohol would rank as the fifth most harmful drug after heroin, cocaine, barbiturates and methadone, he said in a briefing paper for the Centre for Crime and Justice Studies at King's College London.
Tobacco would come ninth on the list and cannabis, LSD and ecstasy "while harmful, are ranked lower at 11, 14 and 18 respectively". The ranking is based on physical harm, dependence and social harm.
"No one is suggesting that drugs are not harmful. The critical question is one of scale and degree," said Nutt, the chairman of the government's Advisory Council on the Misuse of Drugs.
He added: "We have to accept young people like to experiment, with drugs and other potentially harmful activities, and what we should be doing in all of this is to protect them from harm at this stage of their lives.
"We therefore have to provide more accurate and credible information. If you think that scaring kids will stop them using, you are probably wrong."
Nutt criticised ministers for their decision to upgrade the classification of cannabis in January from class C, which includes tranquillisers and some painkillers, to the higher class B alongside amphetamines.
The decision, which increases the penalties to a maximum 14 years in jail for dealing and five years for possession, was against scientific advice and came just five years after cannabis had been downgraded from class B to C.
Nutt said such policies "distort" and "devalue" research evidence and lead to mixed messages to the public.
While he acknowledged that cannabis was "harmful", he said its use does not lead to major health problems. Users faced a "relatively small risk" of psychotic illness compared to the risks of smokers contracting lung cancer.
Nutt caused controversy earlier this year by saying that taking ecstasy was no more dangerous than horseriding, a claim he repeated in his paper.