The process to transform coca leaves into cocaine involves noxious chemicals
Colombian cultivation of the coca plant - used as the raw material in the production of cocaine - has fallen significantly, a UN report shows.
The UN Office on Drugs and Crime said cultivation dropped by 18% in 2008, following a 27% rise in 2007.
The agency said the value of the coca leaf had fallen, causing some farmers to stop growing the crop.
But there was a 4% increase in coca growing in Peru, the report said, and a 9% boost in Bolivia.
In an annual report on the Andean region, the UN agency says the area under cultivation in Peru increased 6%, while in Bolivia the increase was 4.5%.
Last year's survey had reported an increase of 27% in coca cultivation in Colombia.
The UN says that cocaine production in Colombia - as distinct from coca cultivation - was also down, 28% compared with 2007.
It says local officials have "seriously disrupted" drug trafficking, seizing 200 tonnes of cocaine in 2008, a 57% increase on the previous year.
Cocaine supply is shrinking, along with demand in the major markets of North America, according to Antonio Maria Costa, the executive director of the UN's Office on Drugs and Crime (UNODC).
The value of the coca leaf, the raw ingredient, is said to be falling in Colombia, making it less attractive for farmers.
But Mr Costa warned the governments of Peru and Bolivia not to allow coca cultivation and cocaine production to go unchallenged within their borders.
"Peru must guard against a return to the days when terrorists and insurgents profited from drugs and crime," said Mr Costa.