According to National Cancer Institute  website cancer.gov  the Cannabis plant produces a resin containing  psychoactive compounds called cannabinoids which are a group of terpenophenolic  compounds found in Cannabis species (Cannabis sativa L. and Cannabis indica  Lam.)., activate specific receptors found throughout the body to produce  pharmacologic effects, particularly in the central nervous system and the immune  system. Cannabinoids benefits in the treatment for people with cancer -related  symptoms caused by the disease itself or its treatment.
Cannabis, also known as marijuana, originated  in Central Asia but is grown worldwide today. In the United States, it is a  controlled substance and is classified as a Schedule I agent (a drug with  increased potential for abuse and no known medical use). 
Cannabis use for medicinal  purposes:
Cannabis use for medicinal purposes dates  back at least 3,000 years. It was introduced into Western medicine in the 1840s  by W.B. O’Shaughnessy, a surgeon who learned of its medicinal properties while  working in India for the British East Indies Company. Its use was promoted for  reported analgesic, sedative, anti-inflammatory, antispasmodic, and  anticonvulsant effects.
Cannabis use for medicinal purposes dates  back at least 3,000 years. It was introduced into Western medicine in the 1840s  by W.B. O’Shaughnessy, a surgeon who learned of its medicinal properties while  working in India for the British East Indies Company. Its use was promoted for  reported analgesic, sedative, anti-inflammatory, antispasmodic, and  anticonvulsant effects.
In 1937, the U.S. Treasury Department  introduced the Marihuana Tax Act. This Act imposed a levy of one dollar an ounce  for medicinal use of Cannabis and one hundred dollars an ounce for recreational  use. 
Physicians in the United States were the principal opponents of the  Act. AMA believed that objective evidence that Cannabis was addictive was  lacking and that passage of the Act would impede further research into its  medicinal worth. In 1942, Cannabis was removed from the U.S. Pharmacopoeia  because of persistent concerns about its potential to cause harm.
In 1951, Congress passed the Boggs Act,  which for the first time, included Cannabis with narcotic drugs. In 1970, with  the passage of the Controlled Substances Act, marijuana was classified as a  Schedule I drug. Drugs in this category are distinguished as having no accepted  medicinal use. Other Schedule I substances include heroin, LSD, mescaline,  methaqualone, and gamma-hydroxybutyrate.
Despite its designation as having no  medicinal use, Cannabis was distributed to patients by the U.S. government on a  case-by-case basis under the Compassionate Use Investigational New Drug program  established in 1978. Distribution of Cannabis through this program was  discontinued in 1992. In 2010, the U.S. Department of Veteran Affairs approved  marijuana use for patients in states where its medicinal use is  legal.
Source:  http://www.cancer.gov/cancertopics/pdq/cam/cannabis/healthprofessional/page1
But  the U.S. Food and Drug Administration (FDA) has not approved (under big  pharmaceuticals companies) the use of Cannabis as a treatment for any medical  condition till this date. 
Big Pharma’s big contributions to many  legislators means they have many elected officials willing to see things the  drug companies’ way on this, as on many other issues. Even legislators known to  take a strong states’ right stance on other issues, such as offshore drilling,  somehow find themselves standing up for federal oversight on this topic. For  example, Sen. Richard Burr (R-N.C.), who holds the dubious distinction of the  being the member of Congress who has accepted the largest amount from  pharmaceutical firms, has taken a stance against state legalization of  marijuana. 
If you have a politician in your state who  speaks out against medical marijuana, you may want to look into his/her ties to  Big Pharma. As the American Independent observes, the pharmaceutical giants’  strategy as regards marijuana seems to be “demonize it, prosecute it, shut it  down, then grab the market.” Let your friends and family know that many of those  who fight against medical marijuana are not, as they may prefer to present  themselves, taking a pro-family stance against drug addiction, but shills for  multinational drug corporations who want to keep all drug profits in their  coffers.
The idea that a drug can be denounced as evil in one context but  hailed as a medical miracle if sold within the pharmaceutical system is nothing  new.