More than 90 percent of the legal marijuana products offered in medical dispensaries are much stronger than what clinical studies have shown that doctors recommend for chronic pain relief, according to new research.

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"We know that high-potency products should not have a place in the medical realm because of the high risk of developing cannabis-use disorders, which are related to exposure to high THC-content products," said the study's lead author, Alfonso Edgar Romero-Sandoval from Wake Forest School of Medicine in the US. Several studies showed that levels of up to five percent tetrahydrocannabinol (THC) - the main psychoactive compound in marijuana that provides pain relief as well as intoxication - were sufficient to reduce chronic pain with minimal side effects.

The goal of the study, published in the journal PLOS ONE was to evaluate the advertised THC and cannabidiol (CBD) content of legal cannabis products to determine their suitability for medicinal use and to compare the potency of the products offered in medical and recreational programmes.

The researchers recorded the concentrations of THC and cannabidiol (CBD) - the non-euphoric compound in marijuana - in all plant cannabis products provided by legal dispensary websites and compared them between or within the states in the study: California, Colorado, Maine, Massachusetts, New Hampshire, New Mexico, Rhode Island, Vermont and Washington.

A total of 8,505 cannabis products across 653 dispensaries were sampled.

The researchers found that most of the products offered in the medical dispensaries in the study had more than 10 percent THC and that many had 15 percent or more, the same as what is available in products at recreational dispensaries. "This is problematic because between 60 percent and 80 percent of people who use medical marijuana use it for pain relief," Romero-Sandoval said.

The higher the concentration of THC the greater risk, not only for developing dependency but also for developing tolerance more quickly, which means higher and higher concentrations might be needed to get the same level of pain relief, the study said.

"Better regulation of the potency of medical marijuana products is critical. The FDA regulates the level of over-the-counter pain medications such as ibuprofen that have dose-specific side effects, so why don't we have policies and regulations for cannabis, something that is far more dangerous?," Romero-Sandoval said.