Smoking cannabis could suppress the immune system and raise the risk of cancer, according to new research by the University of South Carolina.
Tests done on mice showed that the drug caused a production of cells that weaken the body’s immune system, making it more vulnerable to cancer.
The research was reported in the European Journal of Immunology.
The US study found that cannabinoids, active compounds in cannabis, triggered biological pathways to generate “massive numbers” of the cells, known as myeloid-derived suppressor cells (MDSCs).
MDSCs normally act as a safety brake on the immune system to stop its battle against disease from spiralling out of control.
But in the case of cancer, they may actually make it easier for tumours to grow.
A larger amount of cells could also leave the body open to infection with germs such as those that cause pneumonia.
Lead researcher Dr Prakash Nagarkatti, from the University of South Carolina, said: “These results raise interesting questions on whether increased susceptibility to certain types of cancers or infections caused from smoking marijuana results from induction of MDSCs.
“MDSCs seem to be unique and important cells that may be triggered by inappropriate production of certain growth factors by cancer cells or other chemical agents such as cannabinoids, which lead to a suppression of the immune system’s response.”
In 2004 the Government sparked a storm of protest when it downgraded cannabis from a class B drug to a class C.
Weakening the law proved disastrous leading to a surge in addictions and mental illness.
The drug was restored to the stronger class B classification in 2008 in response to pressure from judges, police, parents and mental health experts.
In the three years after the law was weakened the number of cannabis addicts receiving NHS treatment doubled.
The reclassification was also accompanied by a surge in the number of children aged 15 and under being treated for mental illness.
And a study lasting 27 years involving 50,000 people showed that smoking cannabis trebles the risk of a young person developing schizophrenia.
Last February a UN report revealed that UK teenagers were the worst in Europe for cannabis abuse.
Almost half of 15 and 16-year-olds admitted to using cannabis and up to a third of adults in the UK confessed to taking the drug.
Professor Hamid Ghodse, president of the International Narcotics Control Board, said that weakening the law in 2004 had “given the wrong message to young people in the UK and around the world”.