(CNN)
What are the most addictive drugs? This question seems simple, but the answer depends on whom you ask.
From the points of view of different researchers, the potential for a drug to be addictive can be judged in terms of the harm it causes, the street value of the drug, the extent to which the drug activates the brain's dopamine system, how pleasurable people report the drug to be, the degree to which the drug causes withdrawal symptoms, and how easily a person trying the drug will become hooked.
There are other facets to measuring the addictive potential of a drug, too, and there are even researchers who argue that no drug is always addictive. Given the varied view of researchers, then, one way of ranking addictive drugs is to ask expert panels.
In 2007, David Nutt and his colleagues asked addiction experts to do exactly that -- with some interesting findings.
1. Heroin
Opioids and overdoses: 4 things to know
Nutt et al.'s experts ranked heroin as the most addictive drug, giving it a score of 3 out of a maximum score of 3. Heroin is an opiate that causes the level of dopamine in the brain's reward system to increase by up to 200% in experimental animals. In addition to being arguably the most addictive drug, heroin is dangerous, too, because the dose that can cause death is only five times greater than the dose required for a high. Read more