The chemical in cannabis that produces a high may help to combat the spread of cancer, research suggests.
Cannabis has several medical applications
Scientists have discovered the active ingredient, delta-9-tetrahydrocannibol can block the spread of gamma herpes viruses.
The viruses are linked to an increased risk of the cancers Kaposis sarcoma, Burkitts lymphoma and Hodgkins disease.
The research, by the University of South Florida, is published in the online journal BMC Medicine.
Gamma herpes viruses are different from the herpes simplex viruses responsible for cold sores and genital herpes.
Among those that have been associated with an increased risk of cancer is Kaposis Sarcoma Associated Herpes Virus.
Once infected, it is almost impossible to get rid of the virus as it lies dormant for long periods within white blood cells.
However, the virus can snap back into action, and suddenly begin to replicate itself, bursting out of the cells to infect others. Once a cell has been infected the chances that it will become cancerous are increased.
The South Florida team found that this sudden reactivation was prevented if infected cells were grown in the presence of THC.
Cells infected with a mouse gamma herpes virus normally died when the virus
reactivated. But they survived when cultured with the cannabinoid compound, and thus the spread of the virus - and the potential spread of cancer - was blocked.
The researchers were able to show that THC specifically blocked the gamma herpes viruses - it had no impact at all on the cold sore virus herpes simplex-1.
They hope their findings will lead to the development of new drugs to neutralise the threat of the viruses.
However, lead researcher Dr Peter Medveczky said more work was needed, and stressed that it would not be sensible for people with cancers associated with gamma herpes viruses to start smoking cannabis.
He said THC was known to suppress the immune system - which could do more harm than good to patients whose immune system was often already weakened.
Dr Medveczky believes THC blocks replication of the gamma herpes viruses by targeting a gene they all carry called ORF50.
A spokesperson for Cancer Research UK warned that the results should be treated with caution.
"These are very preliminary results and it is far too early to say whether the findings will lead to practical strategies for preventing and treating cancer."