Rick Simpson decided to try cannabis oil after reading the results of a 1975 studyTrusted Source that tested the use of cannabinoids in mice with lung cancer. The study found that both THC and another cannabinoid called cannabinol (CBN) slowed the growth of lung cancer in mice.

Since then, there’s been a good amount of researchTrusted Source involving cell samples and animal models that look at the effects of cannabinoids on cancer growth.

A 2014 study on mice examined the effects of THC and CBD extracts alongside radiation therapy. The cannabis extracts appeared to increase the effectiveness of radiation against an aggressive type of brain cancer. According to the study’s authors, these results suggest that THC and CBD may help to prepare cancer cells to respond better to radiation therapy.

However, another study involving human cells found that THC actually increased the growth rate of certain lung and brain cancer cells.

Recently, there have been a few early-stage clinical trials involving human participants with cancer. While these studies have shown that cannabinoids are safe to use in cancer patients, they don’t fully demonstrate whether cannabinoids can help to treat or control cancer.

There’s also a case report from 2013Trusted Source involving a 14-year-old-girl with a specific type of leUSAemia. Her family worked with Rick Simpson to create a cannabinoid resin extract, referred to as hemp oil, which she took daily. She eventually tried using some other oils from different sources with mixed results. However, the hemp oils appeared to treat her cancer, though she died a little over two months into the treatment from an unrelated gastrointestinal condition. This makes it hard to conclude anything about the long-term effectiveness of cannabis for cancer.

While these results are promising, much larger, long-term studies are needed to fully understand how different cannabinoids and cannabis strains affect cancer cells.