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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.

There is some suggestion of a genetic role, an inherited predisposition towards addiction.

Abuse and neglect during childhood have been linked, as has growing up in a home where there is drug abuse. Other factors such as mental illness, poverty and poor education have been implicated.

However there are clearly many cases which do not fit any of 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."