CANNABIS FOR IOP?

Table 1: Non-lethality of marijuana usage compared to other 'recreational' agents. Courtesy of Marc F Lieberman MD

The growing “medicalization” of cannabis, by legalising its use on a doctor’s order, pressures physicians to prescribe marijuana and its derivatives – despite limited evidence of its effectiveness.

However, the safety and effectiveness evidence that does exist suggests that marijuana is a poor choice for glaucoma treatment, Marc F Lieberman MD told Glaucoma Day at the 2014 American Academy of Ophthalmology annual meeting in Chicago, USA. He currently serves as Director of Glaucoma Services at California Pacific Medical Centre, and Clinical Professor of Ophthalmology at the University of California San Francisco, USA. The benefits simply do not justify the risks, particularly since many safer and more effective treatments are readily available, he said.

While early studies show marijuana lowers intraocular pressure (IOP) in many eyes, the effect was modest at best, lasting but a few hours. But marijuana’s potential risks, both medical and social, can be substantial and potentially life-long, Dr Lieberman said. Chronic use beginning in adolescence can lead to irreversibly impaired cognitive capacities – and marijuana’s illegality places users at risk for the stigma of criminal conduct and incarceration. Yet compared to other, widely available ‘recreational’ agents, marijuana is fortunately not lethal. (See Table 1)

As such, marijuana as glaucoma therapy violates the Hippocratic dictum to first do no harm, Dr Lieberman said. “In my personal practice I educate patients about the risks, advocate evidence-based regimens, and refrain from prescribing,” he added.

Myth becomes public policy

Despite dozens of recent US state and local statutes permitting its medical or recreational use, US federal law still classifies marijuana as a “Schedule I” agent, incorrectly equating marijuana as having the same high potential for addiction and absence of medical value, as cocaine and heroin. Dating from the 1930s, these prohibitions were politically driven - with racist and xenophobic overtones and without a shred of scientific substantiation, Dr Lieberman said.

Yet many adults with direct experience relate recreational marijuana use to that of alcohol and tobacco. In the past 50 years over 20 million Americans have been arrested, and over $US100 billion spent on prohibition. The American government’s hysterical demonization of marijuana’s risks, plus mass incarceration of minor marijuana offenders, have shredded the credibility of medical-legal authorities, Dr Lieberman said. Its illegal status has also prevented necessary research.

The evidence

Marijuana’s available potency is on an upward trajectory: from four per cent THC-concentration in 1995 to nearly 20 per cent in 2014, Dr Lieberman said. More powerful delivery systems by vapourization (“dabbing”) of oils and waxes of potent marijuana concentrates are readily available. That the plant contains many poorly understood active chemicals, and that its IOP-lowering mechanisms remain unknown, further complicate the challenges of medical studies and recommendations.

And despite public perception, chronic marijuana use is not benign, Dr Lieberman said. About nine per cent of users develop long-term dependency. “At the end of 2014 we have unanimous consensus among many ophthalmological societies that there is no current role for the use of cannabis agents in the management of glaucoma,” Dr Lieberman said.

Marc F Lieberman: sfdrmarc@gmail.com