Researchers have found fresh evidence to suggest that more potent strains of cannabis are at least partly to blame for the number of people seeking help from drug treatment programmes.

Scientists at King’s College London drew on data from the Netherlands to show that admissions to specialist treatment centres rose when coffee shops sold increasingly more potent cannabis, but fell again when the cannabis weakened.

The work is the first to investigate how admissions to drug treatment programmes rise and fall in line with the strength of cannabis available to users. It found that changes in demand for treatment typically lagged five to seven years behind changes to cannabis strength.

“This is the first study to provide evidence for an association between changes in potency and health-related outcomes,” said Tom Freeman, an addiction scientist at King’s.

The demand for specialist treatment among cannabis users has risen steadily in recent years, with more people now citing the drug on admission than any other illicit substance. In Europe, the number of first-time referrals for cannabis rose 53% from 2006 to 2014.

Cannabis plants produce more than 100 active compounds called cannabinoids but THC, or delta-9 tetrahydrocannabinol, is largely responsible for the drug-related high. A second compound called CBD, or cannabidiol, appears to reduce some of the mental health risks linked to heavy cannabis use by counterbalancing the effects of THC.

Many countries have seen far stronger cannabis come on to the market in the past few decades. A major survey in the US found that the strength of illicit cannabis rose from an average of 4% THC in 1995 to 12% in 2014. After two years of legal sales in Washington state, cannabis extracts containing nearly 70% THC now make up one-fifth of the market, researchers found last year. In Britain, the Home Office has not recorded cannabis strength since 2008 when high-strength skunk, containing 15% THC, accounted for 80% of the market.

In work funded by the Society for the Study of Addiction, Freeman and others studied data gathered by the Trimbos Institute, a non-profit mental health and addiction organisation in the Netherlands. Each year, the institute conducts anonymous tests on cannabis for sale at a random selection of coffee shops in the country.

Writing in the journal, Psychological Medicine, the researchers show that THC levels in cannabis soared from an average of 8.6% to 20.4% from 2000 to 2004, then slowly fell to 15.3% by 2015. When the researchers looked at the impact on drug treatment programmes, they found that first-time cannabis admissions nearly quadrupled from seven to 26 per 100,000 inhabitants from 2000 to 2010, and then dropped to less than 20 per 100,000 inhabitants in 2015. It means that for every 1% increase in THC, about 60 more people entered treatment.

“We see a rapid increase in THC between 2000 and 2004 followed by a slower decline, and then you see a very similar profile in drug treatment admissions,” Freeman said. The rise in cannabis potency was one of a number of factors driving admissions to specialist drug services.

Val Curran, professor of psychopharmacology at UCL, said: “This adds to a growing number of scientific studies which suggest rising THC potency of cannabis is associated with greater incidence of mental health problems including addiction and possibly psychosis.”

But she added that stronger cannabis was not solely responsible for increasing demand for drug treatment. “Other factors include the marked decrease in levels of cannabidiol (CBD) in cannabis. There is evidence that CBD can protect against some mental health harms of THC,” she said.

Ian Hamilton, a mental health lecturer at the University of York, agreed that other factors beyond the potency of the drug were important. “It is possible that seeking help for problems with cannabis has become more acceptable by users and treatment providers. Over the same period that cannabis referrals to treatment have been increasing, referrals for problems with opiates such as heroin have been in decline. So although cannabis has traditionally been viewed as relatively benign by treatment workers they may now be more inclined to offer support,” he said.