Why do some people become addicts?

Amy Winehouse's struggle with drink and drug addiction was well known, reflected in her music and widely reported in the media.

But how much do we understand addiction?

What causes it and why do some people become addicts while others do not?

Addiction is naturally associated with drink and drugs, but that is not the whole story.

The NHS points out that people can "become addicted to anything, from gambling to chocolate".

First contact

Addiction has to start with exposure, and at some point casual use shifts to dependence.

Dr Gillian Tober, president of the Society for the Study of Addiction, said all addiction has to start with first use.

"It is usually for social reasons - boyfriend, girlfriend, group of friends - it's usually not pleasant but there is a social reward."

This then becomes reinforced. "People say their first cigarette is disgusting. Some say never again, some break through and reveal the pharmacological effect."

Drugs directly feed the reward circuitry of the brain, and even in cases such as gambling the brain can learn to look forward to the thrill.

The brain adapts to the drug, becomes tolerant to it and demands more each time. Physiological dependence - addiction - emerges.

Resisting addiction

But not everyone becomes addicted. A great many people drink, even fewer are heavy drinkers, and even fewer become dependent.

Ilana Crome, a professor of addiction psychiatry at Keele University, said great progress had been made in recent years in understanding why that is.

"We're beginning to understand the variety of mechanisms in the addictive process, but do we know exactly what causes addiction? We don't.

"It seems to touch the very essence of behaviour, making it very difficult to research and understand."

Doctors cannot point to a 'single cause' of why addictions develop. There are however some risk factors.

The chair of the Faculty of Addictions at the Royal College of Psychiatrists, Dr Owen Bowden-Jones, puts the risks into three categories.

"One way to describe addiction is to think about it as a disorder with biological, psychological and social aspects."

He said that research suggests "people who are vulnerable to addiction may be 'wired' differently" particularly in the brain's orbito-frontal cortex.

"This part of the brain is involved in the weighing up of the pros and cons of a particular action, in other words, decision making."

Psychological trauma, such as through childhood neglect or bereavement, is common, he said.

On the social level he lists living where drugs are easily available or having friends who are addicted as well as poor housing and social deprivation.

However there are clearly many cases which do not fit these risk factors.

Harry Shapiro, from the charity Drug Scope, said addiction was a "complicated phenomenon with a combination of risk factors".

He said it was "impossible to pick people most likely to become addicted, it's such an individual thing."

Prof Crome said: "We can't predict exactly who will become addicted, but many people who are from a difficult background who might be predicted to develop a problem don't and that is a fascinating thing."