A recent study conducted at the University of Bath by the Addiction and Mental Health Group looked at the THC and CBD levels of over 80,000 samples of herbal cannabis and cannabis resin, aka hash, confiscated in the UK, New Zealand, Netherlands, Italy, France, and Denmark, with the majority coming from the US. The study found that from 1970 to 2017, international cannabis markets have shown an increase in delta‐9‐tetrahydrocannabinol (THC) concentration while levels of cannabidiol (CBD) remained stable. This increase in THC was greater in cannabis resin than in herbal options where they noted levels rose 0.29% each year.
However, the increase in herbal cannabis is thought by the researchers to be due to an increase in the popularity of sinsemilla, which has a higher THC level, over traditional herbal cannabis with lower levels. Additionally, they believe that the THC increase and steady CBD level found in hash products is due to the increase of THC-rich ingredients during resin production.