Medicinal cannabis may not ease the sleep problems of people with chronic pain, a study has suggested.
Scientists in Israel believe frequent users of the drug build up tolerance to its effects, rendering it useless after time.
Researchers assessed the sleep quality and pain levels of 128 people being treated at a specialist clinic.
They found cannabis users were less likely to wake up during the night, compared to those who did not use the drug.
But over time, the benefit of cannabis on waking in the night was reversed, with the drug being associated with waking up more often.
Cannabis users also found it harder to fall asleep than those who steered clear of the drug, according to the study.
This indicated that tolerance to the beneficial effects of the drug may develop with further use, the researchers said.
Medicinal cannabis is not legally prescribed for sleep problems in the UK. Limited prescriptions are given out for cannabis-based medications which treat illnesses including multiple sclerosis and epilepsy.
In the US, some states allow doctors to use prescribe medicinal cannabis to treat some conditions.
It is also legal for recreational use in 11 states, meaning that people are free to use it to try to treat their sleep problems.
The latest study was published in the British Medical Journal’s Supportive and Palliative Care journal.
Study author Dr Sharon Sznitman, from the University of Haifa, said the findings may ‘signal development of tolerance’.
She and her colleagues wrote: ‘These findings have large public health impacts considering the ageing of the population, the relatively high prevalence of sleep problems in this population, along with the increasing use of medicinal cannabis.’
The scientists assessed the sleep quality and pain scores of 128 people over the age of 50 being treated at a specialist clinic.
Sixty-six of them used cannabis to manage their sleep problems, while a further 62 did not.
One in four (24 per cent) said they always woke up early and weren’t able to get back to sleep.
And one in five (20 per cent) said they always found it difficult to fall asleep. Another 27 per cent said they woke up during the night.
Cannabis users had used the drug for an average of four years, consuming around 31 grams a month.
Most (69 per cent) smoked it, while around 20 per cent used either using cannabis oil or vapour.
Despite waking up less during the night, cannabis users did not get to sleep more quickly or wake up early less often.
The scientists took into account other factors which might have influenced people’s sleep.
These included the average amount of pain people were experiencing, their use of sleep aids or antidepressants, as well as age and gender.
However, the researchers did say the study was only observational and so could not establish a causal link.
They also recognised there were not many people who took part in the study and the time of day when they had cannabis was not recorded.