Most cannabis being sold illegally in the UK is super-strength skunk linked to a higher risk of psychotic mental health episodes, an analysis of 995 samples seized by the police suggests.
In 2016, 94% of police seizures were high-potency marijuana, compared to 85% in 2008 and 51% in 2005.
The drug contains more of the psychoactive ingredient THC than some other types of cannabis, such as hash.
Researchers from King's College London say users should be warned of this.
Types of cannabis
There are three main types of street cannabis – hash (hashish or resin), herbal cannabis (weed, grass or marijuana) and high-potency cannabis or skunk.
Hash is made from the resin of the plant, while herbal cannabis is made from the dried leaves and flowering parts of pollinated cannabis plants.
Skunk is made from from unpollinated cannabis plants which naturally contain higher levels of THC – the substance that gives recreational users the 'stoned' feelings they seek from the drug, but can also cause nasty side effects, including paranoia and hallucinations.
Hash and herbal cannabis are considered to be milder than skunk. That's because they contain higher levels of a substance called CBD (cannabidiol) which experts say works as an anti-psychotic and counteracts some of the negative effects of THC (tetrahydrocannabinol).
How risky is skunk?
It's argued that cannabis with high levels of THC and no or very low CBD can lead to people developing psychiatric issues.
The skunk examined by the researchers from King's College London was high potency – about 14% THC.
Previous work by the same team, based on a study of 780 people, suggests the risk of psychosis is three times higher for users of potent "skunk-like" cannabis than for non-users.
The use of hash, a milder form of the drug, was not associated with increased risk of psychosis.
According to the Royal College of Psychiatrists, there is sufficient evidence to show that people who use cannabis, particularly at a younger age, such as around the age of 15, have a higher than average risk of developing a psychotic illness, including schizophrenia or bipolar disorder.
What did the study find?
The researchers say their latest work, published in the journal Drug Testing and Analysis, is the first comprehensive survey of cannabis strength published in the UK for nearly a decade.
They analysed police seizures of cannabis from London, Kent, Derbyshire, Merseyside and Sussex in 2005, 2008 and again in 2016.
Skunk was the dominant street drug over this time period, while the availability of weaker cannabis resin went down – from 43% in 2005 and 14% in 2008, to 6% in 2016.
THC levels in skunk remained fairly constant over the decade.
Cannabis and the law
Cannabis is a Class B drug – it's illegal to possess, give away or sell.
Possession is illegal whatever you're using it for, including pain relief. The penalty for possession is up to five years in prison.
Supplying can get you up to 14 years and an unlimited fine. Giving it to friends, even if they don't pay, is considered as supplying.
According to Home Office statistics, cannabis was the most commonly used drug in 2016/17, with 6.6% of adults aged 16 to 59 having used it in the last year. That's around 2.2 million people.
What about medicinal cannabis?
A cannabis-based drug called Sativex has been licensed in the UK to treat MS. It contains THC and CBD.
Doctors could, in theory, prescribe it for other things outside of this licence, but at their own risk.
MS patients prescribed Sativex, who resupply it to other people, also face prosecution.
Another licensed treatment is Nabilone. It contains an artificial version of THC and can be given to cancer patients to help relieve nausea during chemotherapy.
Cannabis oil is a type of medical cannabis that has made the headlines most recently in the UK because of the case of Alfie Dingley, a young boy with severe epilepsy who can suffer up to 30 violent seizures a day.
His mother, Hannah Deacon, has been fighting for the courts to allow her son, who is six, to get the treatment in Britain.