So-called "skunk" now almost completely dominates the cannabis market in the UK, a study suggests.
High-potency strains of the drug accounted for the vast majority of police seizures in a sample, researchers found.
The proportion of confiscations relating to skunk has risen from just over half in 2005 to 94% in 2016, according to the analysis.
Experts raised concerns over the potential mental health implications of a market dominated by strong cannabis.
Senior study author Dr Marta Di Forti, Medical Research Council clinician scientist at King's College London's Institute of Psychiatry, Psychology & Neuroscience, said: "In previous research we have shown that regular users of high-potency cannabis carry the highest risk for psychotic disorders, compared to those who have never used cannabis.
"The increase of high-potency cannabis on the streets poses a significant hazard to users' mental health, and reduces their ability to choose more benign types."
Researchers analysed almost a thousand police seizures of cannabis from London, Kent, Derbyshire, Merseyside and Sussex.
The same areas were last sampled in 2005 and 2008.
In 2016, 94% of police seizures were of high-potency sinsemilla, also known as skunk, compared to 85% in 2008 and 51% in 2005.
The study, published in the journal Drug Testing and Analysis, found the dominance of sinsemilla was mainly due to a reduction in availability of weaker cannabis resin - from 43% in 2005 and 14% in 2008, to just 6% in 2016.
Scientists found the average concentration of THC - the main psychoactive component of cannabis - remained at around 14% for the sinsemilla variety between 2005 and 2016.
Meanwhile, the average THC concentration in resin was higher in the most recent sample, at 6%, compared with 4% eleven years earlier.
The research team are investigating whether changes to the cannabis market in the UK are having a measurable impact on mental health.
Dr Di Forti said: "More attention, effort and funding should be given to public education on the different types of street cannabis and their potential hazards.
"Public education is the most powerful tool to succeed in primary prevention, as the work done on tobacco use has proven."
Official survey findings for England and Wales show cannabis is the most commonly used drug, with 6.6 per cent of adults aged 16 to 59, or around 2.2 million people, having used it in the last year in 2016/17.
This was a similar percentage to 2015/16, but over the longer-term cannabis use has been falling.
The percentage of people having taken the drug in the previous 12 months was 8.2% in 2006/07, and 9.4% in 1996.
Separate data show a sharp fall in the number of possession of cannabis offences recorded by police.
In the 12 months to September, forces logged 80,618 such crimes, down by almost a 10th on the previous year, while the yearly tally was over 100,000 for a decade from April 2005.