This is a question that people often ask but is actually a very difficult one to answer. Addiction is not just one single and simple process, it is a complex process that has both physical and psychological aspects. Nevertheless despite this complexity two attempts have been made to determine the most addictive drug. Independently Dr. Jack E. Henningfield of the National Institute on Drug Abuse and Dr. Neal L. Benowitz of the University of California at San Francisco, ranked six psychoactive substances, nicotine, heroin, cocaine, alcohol, caffeine and marijuana, on the five criteria they felt were most important in addiction.

The first of these criteria was withdrawal. This is defined as the severity of withdrawal symptoms produced by stopping the use of the drug. Both researchers rated alcohol as having the most severe withdrawal symptoms, for example hallucinations and convulsions, and the fact that withdrawal from alcohol is the only one of the included drugs that is potentially fatal.

The second criterion is reinforcement, that is the drug's tendency to induce users to take it again and again. This is influenced by the feelings that taking the drug brings, that is whether it is a pleasurable high or not, obviously if the feelings are negative there will be little incentive to repeat the experience. Again both researchers were in agreement and rated cocaine as the most reinforcing.

The third criterion was tolerance, this is defined as the user's need to have ever-increasing doses to get the same effect. For the first time the researchers disagree with Henningfield rating heroin first and Benowitz rating cocaine.

The fourth criterion is dependence. This is defined as the difficulty in quitting, or staying off the drug, usually measured by the number of users who eventually become dependent. For many dependence is viewed as the hallmark of addiction and how 'addiction' is usually measured by the medical profession. For this criterion both researchers are again in agreement as they rate nicotine highest for dependence. For the other drugs they both rated them in the same order that is highest for dependence, nicotine, then heroin, cocaine, alcohol, caffeine and last marijuana.

The final criterion is intoxication. This is the degree of intoxication produced by the drug in typical use. Again the researchers are in agreement and rate alcohol as the most intoxicating of the drugs.

Given the complexity of the addictive process it is not surprising that there is not a clear 'winner' in all criteria. What some may find surprising is that for both researchers caffeine ranked higher than marijuana on a number of the criteria and indeed Berowitz rates caffeine higher than marijuana for dependence.

These results have been quoted many times by many researchers, commentators and reporters. They are usually interpreted as nicotine, or tobacco smoking, being named the most addictive substance purely on the definition of the difficulty in refraining. What the results do show is that addiction is a complex and multi-facetted activity and that it is impossible to reduce it to a simple metric.