Cannabis-based medicines can cause paranoia and anxiety in some people, a study has suggested.
THC is one of 60 cannabinoids derived from the cannabis plant
Swiss researchers found two out of eight men given drugs containing THC, a chemical extracted from cannabis, developed psychotic effects.
The University of Lausanne team said the public needed to be aware cannabis medicine could have such side effects.
The possibility of using THC to treat multiple sclerosis and pain relief is currently being explored.
GW Pharmaceuticals, the firm granted a UK licence to develop cannabis-based drugs, said: "The levels of THC used in this study would not be used in our medicines.
"We, and everyone else in this field, are aware that THC can lead to psychosis."
During the study, one man reacted to dronabinol, which is licensed for medical use in the US as it is a synthetic, and another to a liquid form of natural THC, the journal BMC Psychiatry reported.
Within hours of taking the drugs orally, both men started displaying psychotic symptoms. Neither had a history of psychiatric problems.
Report author Dr Bernard Favrat said the findings were surprising as the doses used were relatively mild.
"We were not expecting these results. The drugs had a great impact on the people and we have to recognise that drugs which are developed using THC have this risk.
"I am not saying we should not develop them, but just that we need to proceed with caution."
Other recent studies have linked the use of cannabis to mental health problems.
A study by New Zealand scientists, published in March, suggested smoking cannabis virtually doubled the risk of developing mental illnesses such as schizophrenia.
And in December US researchers warned young people that using cannabis led to an increased risk of psychosis later in life.
An MS drug containing 50% THC, which is being developed by GW Pharmaceuticals, is expected to come before regulators later this year. The firm is also developing other drugs containing THC for cancer and back pain.
A spokesman for the firm said the drugs would also contain agents which modify the effects of THC.
The charity DrugScope said that it did not want to see any cannabis-based medicines banned simply because of fears that they may produce side effects.
"There is strong evidence that cannabis can help to treat a range of conditions, and if they can we are in favour of them," it said.
Cliff Prior, chief executive of mental health charity Rethink, added: "This evidence confirms risk of mental health problems from cannabis.
"The Department of Health must act on our campaign for a major education programme, particularly aimed at young people, to inform people of the risks of cannabis."