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Medical cannabis refers to the use of the drug cannabis as a physician-recommended herbal therapy, most notably as an antiemetic.
There are many studies regarding the use of cannabis in a medicinal context. Cannabis was listed in the United States Pharmacopeia from 1850 until 1942. The United States federal government does not currently recognize any legitimate medical use, although there are currently seven patients receiving cannabis for their various illnesses through the Compassionate Investigational New Drug program that was closed to new patients by President George H. W. Bush. Francis L. Young, an administrative law judge with the US Drug Enforcement Agency, in 1988, declared that "in its natural form, (cannabis) is one of the safest therapeutically active substances known." However, smoked cannabis is today not approved by the U.S. Food and Drug Administration (FDA).  Twelve states currently allow for the medicinal use of cannabis.
The term medical marijuana post-dates the U.S. Marijuana Tax Act of 1937, the effect of which made cannabis prescriptions illegal in the United States.
Due to widespread illegality of cannabis as a recreational drug its legal or licensed use in medicine is a controversial issue.
Cannabis has been used for medicinal purposes for over 4,800 years. Surviving texts from ancient India confirm that its psychoactive properties were recognized, and doctors used it for a variety of illnesses and ailments. These included a whole host of gastrointestinal disorders, insomnia, headaches and as a pain reliever, frequently used in childbirth. The ancient Egyptians even used hemp (cannabis) in suppositories for relieving the pain of hemorrhoids.
Cannabis as a medicine became common throughout much of the world by the 19th century. It was used as the primary pain reliever until the invention of aspirin. Modern medical and scientific inquiry began with doctors like O'Shaughnessy and Moreau de Tours, who used it to treat melancholia, migraines, and as a sleeping aid, analgesic and anticonvulsant.
By the time the United States banned cannabis (the third country to do so) with the 1937 Marijuana Tax Act, the plant was no longer extremely popular. Skepticism about marijuana arose in response to the bill. One of the main opponents to the bill was the representative of the American Medical Association.
Later in the century, researchers investigating methods of detecting cannabis intoxication discovered that smoking the drug reduced intraocular pressure. High intraocular pressure causes blindness in glaucoma patients, so many believed that using the drug could prevent blindness in patients. Many Vietnam War veterans also believed that the drug prevented muscle spasms caused by battle-induced spinal injuries. Later medical use has focused primarily on its role in preventing the wasting syndromes and chronic loss of appetite associated with chemotherapy and AIDS, along with a variety of rare muscular and skeletal disorders. Less commonly, cannabis has been used in the treatment of alcoholism and addiction to other drugs such as heroin and the prevention of migraines. In recent years, studies have shown or researchers have speculated that the main chemical in the drug, THC, might help prevent atherosclerosis.
In 1972 Tod H. Mikuriya, M.D. reignited the debate concerning marijuana as medicine when he published "Marijuana Medical Papers 1839-1972".
Later, in the 1970s, a synthetic version of THC, the primary active ingredient in cannabis, was synthesized to make the drug Marinol. Users reported several problems with Marinol, however, that led many to abandon the pill and resume smoking the plant. Patients complained that the violent nausea associated with chemotherapy made swallowing pills difficult. The effects of smoked cannabis are felt almost immediately, and is therefore easily dosed. Marinol (Jojel), like ingested cannabis, is very psychoactive, and is harder to titrate than smoked cannabis. Marinol has also consistently been more expensive than herbal cannabis. Some studies have indicated that other chemicals in the plant may have a synergistic effect with THC.
In addition, during the 1970s and 1980s, six US states' health departments performed studies on the use of medical marijuana. These are widely considered some of the most useful and pioneering studies on the subject,
In May 2001, "The Chronic Cannabis Use in the Compassionate Investigational New Drug Program: An Examination of Benefits and Adverse Effects of Legal Clinical Cannabis" (Russo, Mathre, Byrne et al) was completed. This three-day examination of major body functions of four of the five living US federal cannabis patients found "mild pulmonary changes" in two patients.
Potential health benefits aside, marijuana remains a US Federally controlled substance, making transport across state lines illegal. It has been estimated that an average "marijuana clinic" consumes a pound of cannabis per day, making acquisition a critical challenge. This acquisition may have to resort to illegitimate (i.e., illegal) sources, contributing to crime in communities. This point was illustrated in early 2007, with the tragic murder of Denver area medical cannabis activist Ken Gorman .
Researchers face similar challenges in obtaining medical cannabis for research trial. Recently, the US FDA has approved a number of cannabis research clinical trials, but the Drug Enforcement Agency has not granted licenses to the researchers in these studies.
Early studies on efficacy
A study included 250 patients and compared smoked cannabis to oral THC, under the auspices of the Food and Drug Administration. All participants were evaluated by a medical doctor and had vomiting uncontrolled using at least three alternative antiemetics. Patients chose smoking cannabis or taking an oral THC pill. Multiple objective and subjective standards were used to determine the effectiveness.
Conclusion: Cannabis was far superior to the best available drug at the time of testing, Compazine, and smoked cannabis is clearly superior to oral THC. "More than ninety percent of the patients who received cannabis... reported significant or total relief from nausea and vomiting." No major side effects were reported, though three patients reported adverse reactions that did not involve cannabis alone. The report, dated November 6, 1988, can be read here, though no publication in the peer-reviewed medical literature has been made.
27 patients had failed on other antiemetics therapies, including oral THC.
Conclusion: 90.4% success for smoked cannabis; 66.7% for oral THC. "We found both marijuana smoking and THC capsules to be effective antiemetics. We found an approximate 23% higher success rate among those patients administered smoked marijuana. We found no significant differences in success rates by age group. The major reason for THC capsule failure was nausea and vomiting so severe that the patient could not retain the capsule."
A series of studies throughout the 1980s involved 90–100 patients a year. The study was designed to make it easier for patients to enter the oral THC part of the study. Patients who wanted to smoke cannabis had to be over 15 years old (oral THC patients had to be over 5) and use the drug only in the hospital and not at home. Smoked cannabis patients also had to be receiving rare and painful forms of chemotherapy to qualify.
Conclusion: Despite the bias towards oral THC, the California study concluded that smoked cannabis was more effective and established a safe dosage regimen that minimized adverse side effects. The full text of the study can be seen here.
119 patients that had failed using other antiemetics were randomly assigned to oral THC pills and either standardized or patient-controlled smoking of cannabis.
Conclusion: All three categories were successful — patient controlled smokers at 72.2%; standardized smokers at 65.4%; oral THC at 76%. Failure of oral THC patients was due to adverse reaction (6 out of 18) or failure to improve (9 out of 18); failure of smoking cannabis was due to intolerance for smoking (6 out of 14) or failure to improve (3 out of 14).
Many medical cannabis opponents note that smoked cannabis is harmful to the respiratory system. However, this harm can be minimalized or eliminated by the use of a vaporizer or ingesting the drug in an edible form or other non-smoking modes of delivery like tinctures. Vaporizers are devices that vaporize the active constituents (cannabinoids) and the fragrant aromatic substances in the preparation without combusting the plant material and thus preventing the formation of toxic substances. Studies have shown that vaporizers can dramatically reduce or even eliminate the release of irritants and toxic compounds.
In the United States, the Federal Food, Drug, and Cosmetic Act makes the Food and Drug Administration (FDA) the sole government entity responsible for ensuring the safety and efficacy of new prescription and over-the-counter drugs, overseeing the labeling and marketing of drugs, and regulating the manufacturing and packaging of drugs. The FDA defines a drug as safe and effective for a specific indication if the clinical benefits to the patient are felt to outweigh any health risks the drug might pose. FDA and comparable authorities in Western Europe including the Netherlands, have not approved smoked marijuana for any condition or disease.  Cannabis remains illegal throughout the United States and is not approved for prescription as medicine, although 12 states - Alaska, California, Colorado, Hawaii, Maine, Montana, Nevada, New Mexico, Oregon, Rhode Island, Vermont and Washington - approve and regulate its medical use. (The federal government continues to enforce its prohibition in these states.) However, there are also 2 states, Arizona and Maryland, whose drug laws are favourable towards the medicinal use of marijuana,[clarify] but which still explicitly ban it.
Notable pro- and anti-medical cannabis individuals
The examples and perspective in this section may not represent a worldwide view of the subject. Please improve this article or discuss the issue on the talk page.
Carl Sagan - American astronomer and astrochemist.
Ron Paul - MD, Congressman and Republican candidate in the 2008 United States presidential election. 
Ann Druyan - wife of Carl Sagan, one of the writers of Cosmos: A Personal Voyage.
Drew Carey- Comedian, host of The Price is Right
Willie Nelson - Singer and songwriter.
William F. Buckley - conservative Republican talking head, publisher of the National Review. Author of Free Weeds 29 June 2004, and The Court on High 7 June 2005.
Dennis Peron - co-author of 215, along with Anna Boyce and Scott Imler. Founder of the first Cannabis Club, and in 1995 the 5-story Cannabis Buyers Club at 1444 Market Street (the main street of town) San Francisco.
Gatewood Galbraith - Kentucky attorney; life-long crusader for liberty and human dignity; one of America's leading hemp advocates.
Sister Jane Weirick - among other things Sister Jane was a buyer for Dennis at the San Francisco Cannabis Buyers Club. She worked tirelessly until her death, opening the Hayward Patient's Resource Center.
Dr. Jay Cavanaugh - founder of the American Alliance for Medical Cannabis. Gubernatorial Appointment to California State Board of Pharmacy 1980-82, reappointed 1982-86, reappointed to final Constitutional term 1986-90. Assisted in developing and coordinating drug enforcement against pharmacies, wholesalers, and manufacturers, diverting narcotics. Developed and implemented Recovering Pharmacist Program. Assisted in insuring pharmacist consultation with patients.
Dr. Donald Abrams - Donald Abrams, MD, is Professor of Clinical Medicine at the University of California San Francisco, Chief of Hematology/Oncology at San Francisco General Hospital and Director of Clinical Programs at the Osher Center.
Christopher Hitchens spoke as a proponent of medical marijuana in the 2007 documentary film In Pot We Trust.
John P. Walters - Current Director of the White House Office of National Drug Control Policy of United States.
Mark Souder - U.S. Congressman who filed an amicus brief in support of the U.S. government in Gonzales v. Raich. The federal government may ban the use of marijuana even where states approve its use for medicinal purposes.
Andrea Barthwell - Former deputy director of the White House Office of National Drug Control Policy under George W. Bush.
Paul Clement - Current Solicitor General who argued on behalf of the federal government in Gonzales v. Raich.
Dan Lungren - Former Attorney General of California who presided over crackdown of medical marijuana dispensaries.
Rush Limbaugh - Nationally syndicated radio talk show host. 
Asa Hutchinson - Former Administrator of the Drug Enforcement Administration. 
In the USA, the FDA has approved two cannabinoids for use as medical therapies: dronabinol and nabilone. It is important to note that these medicines are not smoked. Dronabinol is a synthetic THC medication, while nabilone is a synthetic cannabinoid marketed under the brand name Cesamet.
Nausea of cancer chemotherapy that has failed to respond adequately to other antiemetics
Nausea of cancer chemotherapy that has failed to respond adequately to other antiemetics, AIDS wasting
These medications are usually used when first line treatments for nausea fail to work. In extremely high doses and in rare cases there is a possibility of "psychotomimetic" side effects. The other commonly-used antiemetic drugs are not associated with these side effects.
The prescription drug Sativex, an extract of cannabis administered as a sublingual spray, has been approved in Canada for the treatment of multiple sclerosis; this medication may now be legally imported into the United Kingdom and Spain on prescription. Dr. William Notcutt is one of the chief researchers that has developed Sativex, he has been working with GW and founder Geoffrey Guy since the company's inception in 1998. Notcutt states that the use of MS as the disease to study "had everything to do with politics."
On 20 April 2006, The United States Food and Drug Administration (FDA) issued an advisory against medical marijuana stating that, "marijuana has a high potential for abuse, has no currently accepted medical use in treatment in the United States, and has a lack of accepted safety for use under medical supervision. Furthermore, there is currently sound evidence that smoked marijuana is harmful." .
Some prominent American societies have been reluctant to endorse medicinal cannabis.
For example: , the National Multiple Sclerosis Society 
, the American Academy of Ophthalmology  and the American Cancer Society . (Federal Register, 1992).
On June 6 2005, the U.S. Supreme Court handed down a decision which approved the Federal Government's position that federal law permits the prosecution of persons possessing cannabis regardless of the defense that they are medicinal cannabis patients, even in states that exempt its prohibition for medicinal purposes.. 
The Institute of Medicine, run by the United States National Academy of Sciences and funded by the United States federal government, conducted a comprehensive study in 1999 to assess the potential health benefits of cannabis and its constituent cannabinoids. The study concluded that smoking cannabis is not recommended for the treatment of any disease condition, but did conclude that nausea, appetite loss, pain and anxiety can all be mitigated by marijuana. While the study expressed reservations about smoked marijuana due to the health risks associated with smoking, the study team concluded that until another mode of ingestion was perfected that could provide the same relief as smoked marijuana, there was no alternative. In addition, the study pointed out the inherent difficulty in marketing a non patentable herb. Pharmaceutical companies will not substantially profit unless there is a patent. For those reasons, the Institute of Medicine concluded that there is little future in smoked cannabis as a medically approved medication. The report also concluded for certain patients, such as the terminally ill or those with debilitating symptoms, the long-term risks are not of great concern.
In an unpublished 2001 study by the Mayo Clinic, Marinol was shown to be less effective than megestrol acetate in helping cancer patients regain lost appetites.
In 2003, the American Academy of Ophthalmology released a position statement asserting that "no scientific evidence has been found that demonstrates increased benefits and/or diminished risks of marijuana use to treat glaucoma compared with the wide variety of pharmaceutical agents now available." 
Cannabis is in Schedule IV of the Single Convention on Narcotic Drugs, making it subject to special restrictions. Article 2 provides for the following, in reference to Schedule IV drugs:
A Party shall, if in its opinion the prevailing conditions in its country render it the most appropriate means of protecting the public health and welfare, prohibit the production, manufacture, export and import of, trade in, possession or use of any such drug except for amounts which may be necessary for medical and scientific research only, including clinical trials therewith to be conducted under or subject to the direct supervision and control of the Party.
This provision, while apparently providing for the limitation of cannabis to research purposes only, also seems to allow some latitude for nations to make their own judgments. The official Commentary on the Single Convention indicates that Parties are expected to make that judgment in good faith.
Official FDA Statement Regarding Claims of Smoked Marijuana as medicine
Report on and index of marijuana medical studies by Todd Mikuriya, M.D.]
Cannabis-In-Cachexia-Study-Group; Strasser F, Luftner D, Possinger K, Ernst G, Ruhstaller T, Meissner W, Ko YD, Schnelle M, Reif M, Cerny T: Comparison of orally administered cannabis extract and delta-9-tetrahydrocannabinol in treating patients with cancer-related anorexia-cachexia syndrome: a multicenter, phase III, randomized, double-blind, placebo-controlled clinical trial from the Cannabis-In-Cachexia-Study-Group. J Clin Oncol. 2006 Jul 20;24(21):3394-400.
Synthetic THC or low doses of cannabis extract administered orally for cancer-related cachexia (anorexia, weight-loss, emaciation) not better than placebo.
Janet E. Joy, Stanley J. Watson, Jr., and John A Benson, Jr., "Marijuana and Medicine: Assessing the Science Base", Division of Neuroscience and Behavioral Research, Institute of Medicine (Washington, DC: National Academy Press, 1999).
"The accumulated data indicate a potential therapeutic value for cannabinoid drugs, particularly for symptoms such as pain relief, control of nausea and vomiting, and appetite stimulation." and "At this point there are no convincing data to support (the concern that medical marijuana would lead to an increase in recreational use). The existing data are consistent with the idea that this would not be a problem if the medical use of marijuana were as closely regulated as other medications with abuse potential."
Index of studies involving marijuana and multiple sclerosis
Doblin et al., Marijuana as Antiemetic Medicine: A Survey of Oncologists' Experiences and Attitudes," Journal of Clinical Oncology, Vol. 9, No. 7, July 1991.
THC has been found to combat formation of arterial blockages. A random survey of oncologists found that 44% had illegally recommended marijuana for the control of vomiting and that 48% would do so if it were legal; 54% thought it should be available by prescription.
Vinciguerra et al., Inhalation Marijuana as an Antiemetic for Cancer Chemotherapy," The New York State Journal of Medicine, pgs., 525-527, October 1988
56 Patients who had achieved no success with other antiemetics; 72% found success — the study also concluded that smoked marijuana was more effective than oral THC pills.
Chang et al., Delta-9-Tetrahydrocannabinol as an Antiemetic in Cancer Patients Receiving High Dose Methotrexate; Annals of Internal Medicine, Volume 91, Number 6, pg. 819-824, December 1979
A double-blind controlled study found a 72% reduction in nausea and vomiting; the study also concluded that smoked marijuana was more effective than oral THC
Foltin RW, Brady JV, Fischman MW: Behavioral analysis of marijuana effects on food intake in humans. Pharmacol Biochem Behav. 1986 Sep;25(3):577-82.; RW, Fischman MW, Byrne MF: Effects of smoked marijuana on food intake and body weight of humans living in a residential laboratory. Appetite. 1988 Aug;11(1):1-14.; and Greenberg I, Kuehnle J, Mendelson JH, Bernstein JG: Effects of marihuana use on body weight and caloric intake in humans. Psychopharmacology (Berl). 1976 Aug 26;49(1):79-84.
These three studies concluded that marijuana increases appetite.
Sallan SE, Zinberg NE, Frei E 3rd: Antiemetic effect of delta-9-tetrahydrocannabinol in patients receiving cancer chemotherapy. N Engl J Med. 1975 Oct 16;293(16):795-7.
Study concluded that smoked marijuana was more beneficial than synthetic THC for some patients.
Donald P. Tashkin, MD, "Effects of Smoked Marijuana on the Lung and Its Immune Defenses: Implications for Medicinal Use in HIV-Infected Patients"; Journal of Cannabis Therapeutics, Vol. 1, No. 3/4, 2001, pp. 87-102
"Frequent marijuana use can cause airway injury, lung inflammation and impaired pulmonary defense against infection. The major potential pulmonary consequences of habitual marijuana use of particular relevance to patients with AIDS is superimposed pulmonary infection, which could be life threatening in the seriously immonocompromised patient. In view of the immonosuppressive effect of THC, the possibility that regular marijuana use could enhance progression of HIV infection itself needs to be considered, although this possibility remains unexplored to date."
Guy A. Cabral, PhD, "Marijuana and Cannabinoids: Effects on Infections, Immunity, and AIDS"; Journal of Cannabis Therapeutics, Vol. 1, No. 3/4, 2001, pp. 61-85
"However, few controlled longitudinal epidemiological and immunological studies have been undertaken to correlate the immunosuppressive effects of marijuana smoke or cannabinoids on the incidence of infections or viral disease in humans. Clearly, additional investigation to resolve the long-term immunological consequences of cannabinoid and marijuana use as they relate to resistance to infections in humans is warranted."
Ekert H, Waters KD, Jurk IH, Mobilia J, Loughnan P: Amelioration of cancer chemotherapy-induced nausea and vomiting by delta-9-tetrahydrocannabinol. Med J Aust. 1979 Dec 15;2(12):657-9.
In children receiving cancer chemotherapy delta-9-tetrahydrocannabinol (THC) has an antinausea and antivomiting effect.
Sallan SE, Cronin C, Zelen M, Zinberg NE: Antiemetics in patients receiving chemotherapy for cancer: a randomized comparison of delta-9-tetrahydrocannabinol and prochlorperazine. N Engl J Med. 1980 Jan 17;302(3):135-8.
THC seems to be an effective antiemetic in many patients who receive chemotherapy for cancer and for whom other antiemetics are ineffective.
New Studies Destroy the Last Objection to Medical Marijuana
Single Convention on Narcotic Drugs 1961, International Narcotics Control Board.
Dominik Wujastyk, "Cannabis in Traditional Indian Herbal Medicine" in Ana Salema (ed.), Ayurveda at the Crossroads of Care and Cure, Lisbon, Centro de História del Além-Mar, Universidade Nova de Lisboa, 2002, pp.45--73. ISBN 972-98672-5-9. Early pre-publication draft.
^Cannabis Vaporizer Combines Efficient Delivery of THC with Effective Suppression of Pyrolytic Compounds By D. Gieringer et.al. Journal of Cannabis Therapeutics, Vol. 4(1) 2004, 
^Evaluation of a Vaporizing Device (Volcano) for the Pulmonary Administration of Tetrahydrocannabinol. By A. HAZEKAMP, R. RUHAAK, et.al. JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 95, NO. 6, JUNE 2006 abstract
^ Dale Gieringer, "Medical Use of Cannabis in California," in Franjo Grotenhermen, M.D. & Ethan Russo, M.D., ed., Cannabis and Cannabinoids: Pharmacology, Toxicology and Therapeutic Potential, Haworth Press, 2002
^ "American Psychiatric Association Assembly Backs Medical Marijuana Patient Protection." Marijuana Policy Project, press release, November 7, 2007. "In a unanimous vote, the Assembly of the American Psychiatric Association has approved a strongly worded statement supporting legal protection for patients using medical marijuana with their doctor's recommendation."