The plant's psychoactive chemical, THC (tetrahydrocannabinol), is very effective against a range of conditions, from epilepsy, pain relief and depression.
In fact, it had a far more dramatic effect than the 'acceptable' part of the plant, the CBD (cannabidiol), which had very little therapeutic value on its own.
People using the whole plant were seeing an immediate 30 per cent improvement in their symptoms, say researchers from the University of New Mexico.
They monitored therapeutic effects in real time by accessing a cell phone app, ReleafApp, which is used by millions of people taking cannabis products.
The researchers tracked around 20,000 user sessions to discover that THC was the more effective therapeutic agent. "Despite the conventional wisdom, both in the popular press and much of the scientific community, that only CBD has medical benefits while THC merely makes you high, our results suggest that THC may be more important than CBD in generating therapeutic benefits," said Jacob Vigil, one of the researchers.
Although many are using the whole plant for their therapy, the researchers stress it can be addictive and cause short-term problems in cognition and behaviour.