New research published in the peer-reviewed journal Addictionsuggests that contemporary cannabis may be up to 25 per cent stronger than cannabis from the ’70s.

The systematic review examined THC and CBD concentrations in cannabis from studies spanning nearly 50 years and multiple countries, including the US, U.K., France, Denmark, Italy, and the Netherlands.

Using a meta-regression analysis, researchers found that THC concentrations in “herbal cannabis”  increased, on average, by 0.29 per cent each year from 1970 to 2017. While THC concentrations in “cannabis resin” increased 0.57 per cent every year from 1975 t0 2017.

The study suggests that the amount of THC in herbal cannabis increased by 2.9 milligrams each year and 5.7 milligrams for cannabis resin, adding that five milligrams of THC is a “standard dose.”

In the case of CBD, concentrations remained stable in both herbal cannabis and cannabis resin.

The lead author of the study, Dr. Tom Freeman, told the Guardian that cannabis today “differs enormously from the type of drug used by people 50 years ago.”

“During this time attitudes have also shifted. There is now a greater appreciation of its complex interplay with mental health and potential medicinal uses,” he said.

The study notes that the increase of THC in cannabis products underscores the need for guidelines about safe consumption.

“Changes in THC concentrations over time could also influence the efficacy and safety of cannabis used for medicinal purposes, in the absence of standardised dosing information for illicit cannabis products,”  the study notes.