Robert was addicted to heroin for 16 years.

In a spiral of decline he lost his wife, his children, his job and his self esteem.

The quicker the effect of a drug wears off, the more addictive it tends to be

He would wake up feeling sick and spend the day stealing and begging for money to support his habit.

He'd started taking the drug because it gave him pleasure - he'd get a tremendous high, an amazing rush and an escape from the world around him.

Like many users he thought he was in control - until finally he was forced to accept he was addicted.

Brain reward

No-one sets out to be an addict - to be dependent on a particular drug.

Argentinian footballer Maradonna struggled to overcome addiction

Unfortunately with many substances the process is inevitable, and it's only recently scientists have started to understand what's going on when a drug user becomes hooked.

Clare Stamford of University College London, who studies the biochemical process of addiction, says: "People continue to take drugs because they like what the drugs do and want to keep on taking more.

"Unfortunately, people keep taking drugs because if they don't, they get plummeted into a withdrawal syndrome which can be uncomfortable and life threatening."

Drugs like morphine and heroin work by entering a "reward system" in the brain.

They attach themselves to custom-built receptors into which the drug molecules fit like a key into a lock.

Vulnerability

Dr David Best from the UK's National Addiction Centre explains: "People experience an hedonic rush but over a period of time the brain develops a tolerance to the drug, it demands it more and more."

The same people might become addicted to chocolate or sex

The quicker the effect of a drug wears off, the more addictive it tends to be.

But the issue of drug addiction goes far deeper than just a biochemical process in the brain, says Adam Winstock, a lecturer in Clinical psychology at Kings College London.

"It has its roots in both the biology of the individual, how susceptible they are to addiction, but also in the environment they live in.

"Dependency is always going to be an interaction of opportunity and vulnerability."

Take Carol, for example, a member of Narcotics Anonymous in Nairobi, who started taking heroin when she was at college.

She was away from home, lonely and vulnerable. Her friends took heroin, she soon started enjoying it, then she found she couldn't stop.

Addictive personalities

Modern treatments for addiction don't just involve telling people to stop.

The addict's brain always wants more

In many countries heroin addicts are given the drug methadone as a replacement.

In theory users gradually reduce the dose until they're no longer addicted to anything, but many remain on methadone, replacing one addictive drug with another - albeit a legally controlled one.

Dr David Best says: "If we could successfully re-house people in a distant community with a job and a successful relationship then we'd be pretty confident of them staying off drugs."

However, it's a sad fact that with even the most effective and enlightened drugs policy, many people slide back into drug use.

There's evidence to suggest that some people have an "addictive personality" and they just can't stop - the same people might become addicted to chocolate or sex.

There are success stories and even if someone can't give up completely, by being able to seek treatment they're at least regaining some self esteem and an alternative to spending the day trying to get the money for the next quick fix.