One of the arguments used by those promoting medical cannabis is that it has a long history as a therapeutic agent. Written records of its use in China date back to the first century A.D., although oral traditions suggest it was used by as early as 2700 B.C. In India, medical and religious use may have begun around 1000 B.C. and by 1000 A.D. Muslim medical texts mention its use as a diuretic, digestive, anti-flatulent, anti-elliptic and analgesic.

In Europe, cannabis was cultivated for its fibre (hemp) but there was little indication of medicinal uses until the 1840s, when Irish physician William O’Shaughnessy and French psychiatrist Jacques-Joseph Moreau published books on, respectively, its therapeutic and psychoactive potential. 1

With the work of O’Shaughnessy and Moreau, the use of cannabis for medicinal purposes grew; by 1851, for example, it was included in the 3rd edition of the Pharacopoeia of the United States. During the second half of the 19th century over a 100 scientific articles were published on the medical use of cannabis in Europe and the U.S. and various laboratories refined and marketed cannabis extracts or tinctures, including Merck (Germany), Burroughs-Wellcome (England), Bristol-Myers Squibb (United States), Parke-Davis (United States), and Eli Lilly (United States).1 No less than the famed Canadian diagnostician, Sir William Osler, lauded its benefit for the treatment of migraines2 and it’s said, although there is no proof, Queen Victoria was prescribed cannabis to treat menstrual cramps.3

But does a long history mean a treatment is necessarily safe and effective? After all, although investigators may have looked at individual cases or compared those with or without treatment, the “gold standard” of deciding whether a treatment is effective and safe – the randomized controlled trial – did not emerge until a century later.4

A host of treatments were available in the 19th century that we would no longer view as safe, such as laudanum (a tincture of opium with alcohol), cocaine, and strychnine. Mercury, a neurotoxin, was a common treatment for syphilis and other conditions; Abraham Lincoln, for example, was treated for melancholy with a “blue pill” of which the main element was mercury.5 Women used arsenic powder to whiten their faces and the arsenic-based Fowler’s Solution was used to treat a variety of conditions well into the 20th century.6

Given the popularity of medical cannabis in the later 19th century, its decline in the early 20th century was unexpected. A number of factors were involved. First, the active agent of cannabis had yet to be isolated and the potency of tinctures or extracts varied greatly depending upon plant origin, age and preparation methods, making it a difficult substance to prescribe. As well, by the early 20th century other new medicines were available. Many of the indications for cannabis could now be more effectively addressed by a new generation of vaccines (e.g., tetanus), analgesics (aspirin), narcotics and sedatives (chloral hydrate, paraldehyde, barbiturates and injectable morphine).1

But perhaps the greatest challenge to medical cannabis came from an international movement to limit drugs that were seen as highly addictive and a threat to public health. In 1923, William Lyon Mackenzie introduced the Act to Prohibit the Improper Use of Opium and other Drugs, which added to the drugs already on the schedule of controlled substances (opium, morphine, cocaine and eucaine, a medication formerly used as a local anesthetic) an additional three: heroin, codeine and cannabis.

Similar legislation was passed in the UK in 1928, various U.S. states throughout the 1930s, and by the U.S. federal government (the Marihuana Tax Act) in 1937. Although the objective of the legislation was to restrict recreational use, it had a chilling effect upon medicinal use as well: in 1942, for example, cannabis was removed from the Pharacopoeia of the United States. Although cannabis could be accessed for medical and scientific purposes, the security requirements were onerous and, for researchers, costly.

Starting in 1996 in California, a number of jurisdictions passed legislation once again permitting the use of cannabis to treat people with various medical conditions. However, significant gaps remain in our understanding of the indications and contraindications for medical cannabis, its therapeutic impact, recommended dosing, and common and rare adverse effects.

The good news is that contemporary researchers have tools unknown in the 1840s. They include, among others: synthetic versions of cannabinoids and receptor blockers (substances that block a chemical from binding to receptors on a cell) that can be used in animal models; a better understanding of the pharmacology of the many forms of cannabinoids; and more robust research methods such as the randomized controlled trial (RCT), systematic review, and meta-analysis (a means of combining and analyzing data from several RCTs).

If there is systematic and rigorous utilization of these research tools, understanding of medical cannabis has the potential to increase in relatively short order. It’s critical to do so. Public demand for medical cannabis is increasingly rapidly and there’s a danger use could outpace the underlying science. Medicine is full of therapies and approaches that once appeared reasonable and beneficial but under scrutiny later proved to be useless or even harmful (e.g., hormone replacement therapy was once promoted as a means of reducing cardiovascular disease in post-menopausal women but later research showed that it could be detrimental).7

To protect patients, ensure efficient use of healthcare resources, and optimize the use of therapeutic agents, we need a comprehensive evidence base. The Michael G. DeGroote Centre for Medicinal Cannabis Research has been created to address gaps in our understanding of medical cannabis and provide the sort of information – both positive and negative – that is needed to ensure that when it is used, it is safe and effective.

Author Details

The latest scientific evidence on this topic was reviewed by the Centre's leadership team. This evidence brief is written by Corinne Hodgson, DHealth, assessed for accuracy by Medical Advisor Dr. Ramesh Zacharias, MD, a clinician with expertise in chronic pain. There are no conflicts of interest. Questions regarding this piece should be directed to Dr. Ramesh Zacharias (

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