A standard unit system for cannabis should be introduced, similar to alcohol, after its strength has increased by up to 24 per cent in the past 40 years, researchers say.
With a standardised system, users could limit their use and in turn increase drug safety, it has been suggested.
This would work “most easily” in a market where cannabis is legally regulated, the researchers said.
Currently, it remains illegal to possess, grow, distribute, or sell cannabis in the UK, but in some rare cases a prescription for medical cannabis can be issued.
Researchers from the University of Bath combined data from more than 80,000 cannabis samples that had been collected from streets in the USA, UK, Netherlands, France, Denmark, Italy and New Zealand.
They found concentrations of delta-9-tetrahydrocannabinol or THC - the main psychoactive constituent of cannabis - had increased by 14 per cent in herbal cannabis from 1970 to 2017.
In cannabis resin, which is extracted from herbal cannabis, THC concentrations rose by 24 per cent between 1975 and 2017.
But, concentrations of cannabidiol or CBD, which is not intoxicating, did not change over time, according to research funded by the Society for the Study of Addiction, published in the journal Addiction.
Dr Tom Freeman, of the University of Bath, said the rise in cannabis strength highlights the need to implement wider strategies for harm reduction, similar to those used for alcohol, such as standard units and public guidelines on safer consumption limits.
"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," Dr Freeman said.
"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."
Cannabis resin, also known as hash, is often seen as a safer type of cannabis, but the study found that it is now stronger than herbal cannabis.
"As the strength of cannabis has increased, so too has the number of people entering treatment for cannabis use problems," Dr Freeman said.