The estimated number of people using opioids - drugs ranging from opium and derivatives such as heroin to synthetics like fentanyl and tramadol - was 56% higher in 2017 than in 2016, the UN Office on Drugs and Crime said.

The report highlights the scale of the problem despite a drought-related fall in opium production last year in the world's biggest producer, Afghanistan.

Synthetic opioids like fentanyl continue to be the main cause of opioid overdose deaths in the United States, which rose 13 per cent to more than 47,000 in 2017.

"The opioid crisis that has featured in far fewer headlines but that requires equally urgent international attention is the non-medical use of the painkiller tramadol, particularly in Africa," the UNODC 2019 World Drug Report said.

Several West African countries have reported that tramadol is one of the most widely used drugs for non-medical purposes after cannabis.

"In farming communities, there are reports of tramadol being used by humans and fed to cattle to enable them to work under extreme conditions. Others use tramadol as a recreational drug on account of its stimulant and euphoric effects, or to improve sexual stamina." The report said.

Australian Associated Press