Anecdotal reports and some clinical studies suggest that marihuana (Cannabis sativa) is effective in treating a variety of conditions such as glaucoma, migraine, pain, spasticity of multiple sclerosis, anorexia, insomnia, depression, nausea and vomiting. One of the diseases mostly associated to a beneficial effect from marihuana is cancer. Twenty-one states of the United States including the District of Columbia have approved the use of marihuana for cancer and other medical conditions. In Puerto Rico, public debate on criminal penalty removal and medicalization of marihuana has intensified. It is considered essential for health professionals to have strong scientific evidence on the effectiveness and safety of medications or substances when recommending them for treating illness. This article discusses scientific evidence and information provided by prestigious organizations on the effectiveness and safety of marihuana and its derivatives in cancer patients.