Cannabis has been found to help cancer patients with the symptoms that usually accompany cancer such as pain, nausea, wasting, and loss of appetite. In a meta-analysis of 30 clinical studies on the therapeutic use of cannabis for chemotherapy-induced nausea and vomiting, Delta9-THC (dronabinol. AKA marinol) proved superior to modern anti-nausea medication. Additionally, patients showed a clear preference for cannabinoids as anti- nausea  medication over conventional drugs, when receiving chemotherapy. 

Only one clinical trial has ever been published on the effects of Delta9-THC on cancer growth in humans. Doctors administered oral Delta 9-THC to nine patients who experienced tumor progression despite surgical therapy and radiation treatments. The major finding of the study was that Delta 9-THC was safe and did not cause any obvious psychoactive effects in a clinical setting. Furthermore, extensive pre-clinical research clearly indicates that cannabinoids can have tumor-reducing and anti-cancer properties.

In 2009, a clinical trial involving 177 patients with intractable cancer pain and who experienced inadequate relief from opiates, showed remarkable reductions in pain scores from using a cannabis extract which contained THC and CBD. This THC:CBD extract was more effective than an extract containing only THC.

The effects of cannabis may also provide an improvement in mood. In addition to THC, other cannabinoids on the plant such as CBD can inhibit the side effects of THC, and provide relief from anxiety and depression. By contrast, several conventional medications commonly prescribed for cancer patients (phenothiazines such as haloperidol, known as “major tranquillizers”) may produce unwanted side effects such as excessive sedation, flattening of mood, and/or distressing physical “extrapyramidal” symptoms such as uncontrolled or compulsive movements.