Scientists compared wastewater samples taken between 2016 and 2019, 18 months before and after codeine was rescheduled to a prescription drug, according to the study published in Addiction.

It showed codeine concentrations decreased 37 per cent nationally, dropping between 24 and 51 per cent in states and territories, according to combined research by the University of Queensland and the University of South Australia.

Opioids are among the most widely used analgesic medicines globally, with more than 61 million people using them in 2018.

A 2013 study showed Australians took more codeine as a country than the United States, despite having roughly seven per cent of the population.

Codeine sales were responsible for more than 100 deaths each year, according to the Therapeutic Goods Administration.

However, from February 2018 codeine sales were restricted, only available across Australia through a prescription.

“Less availability not only means less chance for codeine to be misused but also changes the perception of codeine being a harmless drug,” UQ researcher Dr Ben Tscharke said.

“Making it prescription-only sends a clear message about how dangerous it is.”

The wastewater samples in South Australia and Tasmania recorded the smallest decreases – averaging 25 per cent – and the Northern Territory the largest decrease of 51 per cent.

Similar reductions were observed in monthly calls to poison call centres, where poisonings halved after the rescheduling of codeine.

Colder climates (ACT, Victoria and Tasmania) had a more significant variance between winter and summer codeine consumption, with more chronic and age-related ailments in winter causing pain.