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What are the most addictive substances? According to Eric Bowman, Lecturer in Psychology and Neuroscience at the University of St Andrews, the answer depends on whom you ask. 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.

Given the variation in answers, one way of ranking addictive drugs is to ask expert panels. In 2007, British neuropsychopharmacologist David Nutt and his colleagues asked addiction experts to do exactly that – with some interesting findings.

1. Heroin

The 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. Heroin also has been rated as the second most harmful drug in terms of damage to both users and to society. In the UK, seaside resorts have the highest rates of death from heroin and morphine, according to figures released from the Office for National Statistics (ONS). The amount of heroin estimated to be imported annually into the UK is between 18-23 tonnes. The vast majority of this is derived from Afghan opium.

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 estimated that between 14-20 million people worldwide use cocaine, and that about 21% of people who try the drug will become dependent on it at some time 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. The amount of cocaine estimated to be imported annually into the UK is between 25-30 tonnes. World Cup mania last summer was reported to have helped drug traffickers smuggle cocaine around the world.

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 and his researchers’ expert panels rated nicotine (tobacco) as the third most addictive substance.

In 2002, the World Health Organization (WHO) estimated there were more than 1 billion smokers and it has been projected that tobacco will kill more than 8 million people annually by 2030.

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.

5. Alcohol

Although legal, alcohol was scored by the expert panel as 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 by 40-360% – 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 alcohol is responsible for more than 5% of all deaths worldwide, or around 3 million a year. For information on any of AlphaBiolabs’ drug testing or alcohol testing solutions – including continuous alcohol monitoring – please visit, call 0333 600 1300 or email us at