It's the most widely used illegal drug in the world, but does cannabis deserve its reputation as being one of the safest?

Not if you take a look at South Africa, one of the UK's biggest providers of high-strength cannabis, where a potent strain of the plant dubbed 'swazi gold' is wrecking havoc on its young population.

As revealed in BBC's Stacey Dooley Investigates, which airs tonight, poverty-stricken grandmothers are growing it, drug mules are risking their lives to smuggle it out of the country, and what stays behind is ruining the lives of South Africa's drug-addled teens.

Confiscated: Reporter Stacey Dooley (left) poses with blocks of densely-packed cannabis which have been intercepted by officials (centre and right) at South Africa's O R Tambo International Airport

Swazi gold is grown in the tiny South African sovereign state of Swaziland, where the climate and nutrient-rich soil is perfect for growing marijuana.

Farmers - many of them 'Grannies' whose children have died and who are now in charge of providing for their grandchildren - have developed the potent strain through cross-pollination, and can now yield double the crop they once could annually.

The result is cannabis which boasts a staggeringly high concentration of THC, the psychoactive agent which produces the high, which finds its way either to the streets of the UK, or the streets of Johannesburg and beyond.

We don't care where it ends up,' one farmer tells reporter Stacey Dooley.

The swazi gold that stays in South Africa is now commonly mixed with heroin to make a highly addictive new cocktail known as nyope.

Nyope contains a heroin concentration of around ten per cent, and often 'fillers' including rat poison and crushed HIV tablets.

Hooked: South African youths as young as 14 are mixing swazi gold with heroin to form nyope, a highly addictive substance that causes long-term mental problems (file photo)

'I smoke until I die,' one young addict based in Johannesburg's Ivory Park tells Ms Dooley. 'I get crazy if I don't get this.'

'We want to stop but if you are addicted you can't just leave,' complains another.

Nyope addicts, many starting out as young as 14, smoke an average of 15 heroin-laced 'joints' per day.

I smoke until I die. I get crazy if I don't get it

They go through stages of painful withdrawal on a daily basis between bouts of being high, with symptoms including restlessness, aching bones and nausea; and most are left with lifelong mental problems as a result of taking the drug.

Ms Dooley meets one 18-year-old nyope addict called Tulu who attends a rehab centre every day in a bid to turn his life around. He has been clean for three days when Ms Dooley meets him, and says he's determined to kick the habit.

Lucrative: Farmers in Swaziland developed the potent strain through cross-pollination, and can now yield double the crop they once could annually (file photo)

Tulu's parents died when he was just 11, leaving him alone to look after his younger sister.

'She's eight, I have to give her something so she can eat at school,' he explains.

It's difficult to know where to start when it comes to cracking down on South Africa's drug trade, Ms Dooley concludes.

Farmers and dealers simply pay off corrupt members of the police force to avoid being arrested on a daily basis, and the story is much the same in airports.

When Ms Dooley visits Johannesburg's O. R. Tambo International Airport, she witnesses one incident of a suitcase packed with cannabis being intercepted, but airport officials admit that more often than not, drugs get through.