Medical marijuana eases pain, anxiety and sleep disorders in elderly people with neurological conditions, according to new research.
After just four months it helps those caused by chronic disorders including Parkinson's disease, multiple sclerosis, motor neurone disease and spinal cord or nerve damage.
About seven in ten (69 per cent) reported some improvement after taking cannabis for four months.
As well as being safe and effective, it also caused a third of patients to reduce their use of powerful opioids that can lead to addiction.
In the US, the super-strength painkillers have killed more than 91,000 people in the past two years.
A huge rise in prescriptions in Britain has led to MPs, senior doctors and drug specialists warned the UK is heading for a similar crisis.
Study author Professor Laszlo Mechtler, of Dent Neurologic Institute, Buffalo, New York, said: "With legalisation in many states, medical marijuana has become a popular treatment option among people with chronic diseases and disorders, yet there is limited research, especially in older people.
"Our findings are promising and can help fuel further research into medical marijuana as an additional option for this group of people who often have chronic conditions."
They were based on 204 over 75s, with an average age of 81, who were enrolled in New York State's Medical Marijuana Program.
Participants took various amounts of THC (tetrahydrocannabinol) and CBD (cannabidiol), the main ingredients in medical marijuana, for an average of four months and had regular checkups.
The former is the psychoactive compound in cannabis that makes users 'high', interacting with receptors in the central nervous system.
The latter does not have this effect, and is thought to help reduce anxiety and inflammation.
They were administered either in liquid form, as a capsule or in an electronic vaporiser.