Smoking cannabis may suppress the immune system and increase cancer risk, according to new research.
The drug triggers the production of cells that weaken the body's resistance to cancer, scientists believe.
A US study found that cannabinoids - active compounds in cannabis - activated biological pathways to generate "massive numbers" of the cells, known as myeloid-derived suppressor cells (MDSCs).
MDSCs are special immune cells that act as a safety brake on the immune system.
They suppress immune responses to prevent them getting out of control, but in so doing are also thought to promote cancer growth.
Cancer patients are known to have increased numbers of MDSCs.
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."
The research is reported in the European Journal of Immunology.
A related study in the same journal, led by Dr Christian Vosshenrich from the Pasteur Institute in Paris, showed that when cancer cells grow they produce a signalling molecule called interleukin-1 beta (IL-1Beta). This also triggered MDSC production, reducing the body's ability to defend itself against cancer.
