Excessive use of cannabis may lead to brittle bones, new research suggests.

Excessive use of cannabis may lead to brittle bones, new research suggests.

Scientists have found that molecules on the surface of bone cells are targeted by cannabis chemicals.

They discovered that drugs which block these cannabinoid receptors may prevent bone loss. But the flip-side to the research is that smoking cannabis is likely to promote osteoporosis.

Professor Stuart Ralston, who led the research at the University of Aberdeen, said: "We hadn't studied cannabis users, but the work we've done would suggest that if you use a lot of cannabis it could stimulate bone-absorbing cells, and that would be bad."

The study's findings are published as ministers prepare to review existing cannabis laws.

Receptors are molecules that act like a "lock" into which other molecules fit. Molecules that affect cells are activated when they bind to specific receptors.

Professor Ralston's team was investigating the way natural cannabinoids in the body attach to receptors to help regulate bone density and turnover.

The "endogenous" cannabinoids seemed to stimulate the absorption of bone - and it was likely that chemicals in cannabis did the same.

Because the molecules have a similar structure, they are likely to bind to the same receptors, the professor said. "It is very likely, almost certain," he said.

Mouse experiments showed that blocking the cannabinoid receptors effectively inhibited bone loss. Conversely, stimulating them with drugs that mimicked the effects of cannabis was detrimental to bone.

"This is an important finding since it demonstrates that the receptors which cannabis acts upon are not only important in the nervous system, but also in the control of bone metabolism," said Professor Ralston, who is now at the University of Edinburgh.

"That compounds which blocked cannabinoid receptors are highly effective at preventing bone loss is particularly exciting, since it shows that these drugs could provide us with a completely new approach to the treatment of osteoporosis." The findings appear in the online edition of the journal Nature Medicine.

Professor Ralston, one of Britain's leading osteoporosis experts, said he saw many patients with bone loss who used drugs of various types. But he pointed out that diet, smoking, and other lifestyle issues may also be involved. "If using cannabis is one of the factors involved, we ought to know about it," he added.

More than 250,000 people in Britain suffer osteoporosis-related fractures each year, with related health costs exceeding £1.7bn. A spokesman for the National Osteoporosis Society said: "This looks to be a new area of research and we are not aware that there has been a lot of research into this issue. It is encouraging from our point of view that new areas are being explored in osteoporosis research."

Health warnings

Cannabis was reclassified from a class B to a class C drug in January 2004. Since then research has revealed new risks.

* A study in Belgium found that cannabis doubles the risk of schizophrenia, hallucination and paranoia among a genetically susceptible group.

* According to the Institute of Psychiatry at King's College London, one in four people carries genes that increase vulnerability to psychosis illness if he or she smokes cannabis as a teenager. Other illnesses such as depression have also been linked to the drug.

* Scientists at Queen's University, Belfast, have warned that use of the drug by men can damage sperm, reducing fertility.

* Researchers in New Zealand have found that heavy cannabis users are ten times more likely to be injured, or to injure others, in car accidents.