Answer: Support is growing for policy based on sound evidence rather than strongly held beliefs or history. The medicinal cannabis debate is a prime example of beliefs trumping evidence.

Anecdotal evidence, though admissible in courts, is typically provided by people who have experienced an effect or observed it in others. Because anecdote lacks corroboration, it is hard to assess scientifically and is no basis for law making.

Instead, primary research, published in reputable journals after peer-review, is considered paramount. In most countries, human medical research on cannabis has been hampered by legal and research funding obstacles, whereas research attempting to identify harms from recreational cannabis has been generously funded.

The resulting "publication bias" has been powerful. Although cannabis is now used medicinally in almost half of the states of the United States, the US Federal government still classifies marijuana as 'Schedule 1', ie it has a high potential for abuse, no currently accepted medical use in treatment, and is unsafe even when used under medical supervision. This means that officially in the US, medicinal cannabis is rated as more dangerous than cocaine.

A spokeswoman for the National Institute of Drug Abuse, an arm of the US government, said, "We don't fund studies that might make cannabis look good, only studies that might make it look bad." US government attitudes strongly influence other countries, including Australia.

Despite this publication bias, sufficient research of adequate quality to justify allowing medicinal cannabis has accumulated, mainly over the last two decades. The number of Medline-indexed publications has doubled each decade, from about 400 in 1990 to about 1600 in 2010 - but more research should have been conducted. Nevertheless policy still has to be based on published research evidence rather than putative results of research that has not been carried out.

Response by Sydney University Professor emeritus professor Laurie Mather and Australian Drug Law Reform Foundation president Dr Alex Wodak. 

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