The strength of cannabis around the world has spiked by nearly 15 percent in the past 50 years, something researchers say is increasing the risk for consumers.
New research led by the University of Bath in the UK found concentrations of THC in herbal cannabis, the main psychoactive compound in the drug and responsible for the mental changes many find enjoyable, including euphoria and relaxation, saw a 14 percent increase between 1970 and 2017, based on street samples collected in New Zealand, Italy, Denmark, France, the Netherlands, the UK, and the US.
University of Bath addiction and mental health group director Tom Freeman says the increase in THC levels is causing more harm.
Sensations of nausea, anxiety, and paranoia can be caused by excessive consumption of THC - otherwise known as "greening out".
"As the strength of cannabis has increased, so too has the number of people entering treatment for cannabis use problems," Dr Freeman said in a statement. "As the strength of cannabis has risen, consumers are faced with limited information to help them monitor their intake and guide decisions about relative benefits and risks.
"The introduction of a standard unit system for cannabis - similar to standard alcohol units - could help people to limit their consumption and use it more safely."
In addition, the research shows THC concentrations in cannabis resin rose by 24 percent between 1975 and 2017.
"As THC concentration varies in cannabis, people partially titrate their THC consumption by adapting their smoking behaviour," says the research published in the Society for the Study of Addiction journal.
"However, this does not fully compensate for differences in THC concentrations and as a result, increasing concentrations of THC in cannabis deliver higher doses of THC to the consumer.
"Increases in THC concentrations have important implications for the health effects of cannabis."
Between 1970 and 2017, researchers analysed nearly 67,000 samples of herbal cannabis and more than 17,300 samples of cannabis resin.
In the study, researchers explain there was a 2.9mg increase in THC levels each year for the typical gram of herbal cannabis. The research, however, found no evidence of change in levels of CBD - a substance found in cannabis that has little if not any psychoactive traits.
"Overall, concentrations of CBD were very low in herbal cannabis but were generally higher in cannabis resin," the study says.
"Concentrations of CBD remained stable in both herbal cannabis and cannabis resin."
New Zealand recently voted down a referendum to legalise cannabis - something that's disappointed advocates of legalising and controlling it.
"The issue of [cannabis] supply remains one of the biggest potential harms in terms of the opportunity for people to graduate or escalate to harsher substances, get bound up in the criminal justice system - and the potential perverse incentives you create when you decriminalise something and don't deal with that supply chain," Green Party MP Chlöe Swarbrick told Newshub Nation last month.