(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
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.
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Heroin also has been rated as the second most harmful drug in terms of damage to both users and to society. The market for illegal opiates, including heroin, was estimated to be $68 billion worldwide in 2009.
2. Cocaine
Cocaine directly interferes with the brain's use of dopamine to convey messages from one neuron to another. In essence, cocaine prevents neurons from turning the dopamine signal off, resulting in an abnormal activation of the brain's reward pathways. In experiments on animals, cocaine caused dopamine levels to rise more than three times the normal level. It is estimatedthat between 14 million and 20 million people worldwide use cocaine and that in 2009 the cocaine market was worth about $75 billion.
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Crack cocaine has been ranked by experts as being the third most damaging drug and powdered cocaine, which causes a milder high, as the fifth most damaging. About 21% of people who try cocaine will become dependent on it at sometime in their life. Cocaine is similar to other addictive stimulants, such as methamphetamine -- which is becoming more of a problem as it becomes more widely available -- and amphetamine.
3. Nicotine
Nicotine is the main addictive ingredient of tobacco. When somebody smokes a cigarette, nicotine is rapidly absorbed by the lungs and delivered to the brain. Nutt et al's expert panels rated nicotine (tobacco) as the third most addictive substance.
More than two-thirds of Americans who tried smoking reported becoming dependent during their life. In 2002 the WHO estimated there were more than 1 billion smokersand it has been estimated that tobacco will kill more than 8 million people annually by 2030. Laboratory animals have the good sense not to smoke. However, rats will press a button to receive nicotine directly into their bloodstream -- and this causes dopamine levels in the brain's reward system to rise by about25% to 40%.
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4. Barbiturates ("downers")
Barbiturates -- also known as blue bullets, gorillas, nembies, barbs and pink ladies -- are a class of drugs that were initially used to treat anxiety and to induce sleep. They interfere with chemical signalling in the brain, the effect of which is to shut down various brain regions. At low doses, barbiturates cause euphoria, but at higher doses they can be lethal because they suppress breathing. Barbiturate dependence was common when the drugs were easily available by prescription, but this has declined dramatically as other drugs have replaced them. This highlights the role that the context plays in addiction: if an addictive drug is not widely available, it can do little harm. Nutt et al's expert panels rated barbiturates as the fourth most addictive substance.
5. Alcohol
Although legal in the US and UK, alcohol was scored by Nutt et al.'s experts 1.9 out of a maximum of 3. Alcohol has many effects on the brain, but in laboratory experiments on animals it increased dopamine levels in the brain's reward system by40% to 360% -- and the more the animals drank the more dopamine levels increased.
Some 22% of people who have taken a drink will develop dependence on alcohol at some point during their life. The WHO has estimated that 2 billion people used alcohol in 2002 and more than 3 million people died in 2012 due to damage to the body caused by drinking. Alcohol has been ranked as the most damaging drug by other experts, too.
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