THC Reduces Eye Pressure, Study Finds

THC is known to lower pressure within the eye, which is one of the largest contributing factors to developing glaucoma.

Despite this, cannabis-based medicines have not yet been adopted in clinical practice as a recommended way of treating dangerously high intraocular pressure (IOP) levels. One of the main reasons for this is the poor corneal penetration of most available topically-administered pharmaceutical formulations of THC.

Now, a new study led by scientists at the Gavin Herbert Eye Institute at the University of California has found that using cannabis products is also associated with a significant drop in IOP.

Specifically, the researchers found that THC blood plasma levels of up to 20 nanograms per milliliter (ng/ml) were strongly correlated with IOP reduction in healthy adults. Levels higher than this delivered diminishing returns.

In light of these findings, the researchers say that more work is needed to determine the direct efficacy of THC in reducing IOP in eyes with ocular hypertension and glaucoma.

Higher plasma THC levels reduce intraocular pressure

Eleven participants (and their 22 eyes) were studied. All of the participants held valid driver’s licenses, had driven at least one thousand miles in the past year, had previous experience with smoking cannabis, and had used the drug at least four times in the previous month. All of the eyes studied were healthy, with no known history of glaucoma or other eye disorders.

During the experiment, participants were given a cannabis cigarette and instructed to smoke it “the way they do at home to get high,” taking at least four puffs within the ten-minute administration window. THC plasma levels were taken prior to smoking, then at around 12, 40, 80, and 120 minutes and every hour for three hours post-administration. Three IOP readings using the FDA-approved noncontact Reichert Ocular Response Analyzer were also taken at these time points and averaged for each eye.

The average peak plasma THC level was 45 ng/mL, recorded at around 12 minutes after smoking. This level declined rapidly to less than 10 ng/mL on average by the hour mark. The average IOP level recorded before smoking was 17.5 millimeter of mercury (mmHg), which decreased by up to 16 percent in the hour after smoking. The IOP levels remained lowered for around four hours after administration, even as plasma THC levels began to decrease.

Statistical analysis revealed that increasing plasma levels up to 20 ng/mL were strongly associated with a decrease in IOP, but levels above this threshold did not correlate with significant further reductions.

“The results of this study indicate that in healthy adult subjects, inhaled THC significantly lowers IOP, and that this effect correlates with plasma THC levels,” the study authors wrote.

“This study further suggests that a peak THC plasma level above 20 ng/mL is not correlated with further IOP reduction, and that nonphysiologic IOP levels are not seen with increasing plasma levels of THC in healthy subjects.”

“Defining the role of cannabis in glaucoma treatment requires further studies to better characterize these effects in different patient populations,” they added. 

Cannabis, IOP, and glaucoma

In glaucoma, the eye’s optic nerve becomes damaged over time and begins to reduce peripheral vision. As the glaucoma advances, this can eventually lead to total blindness; the disease is currently estimated to be the second leading cause of blindness worldwide.

One of the main causes for optic nerve damage is elevated IOP. Research from as early as the 1970s has shown that THC can reduce IOP. However, the poor corneal penetration of topical THC formulations has meant that more traditional treatments have continued to outperform cannabis-based medicines. Improving this corneal penetration and ocular bioavailability is still an active area of research in cannabis science.

Additionally, the mechanisms behind cannabis’ effect on IOP is still not fully understood, though there are several cannabinoid receptors located throughout the eye that may be involved in this effect.

“It has also been suggested that some cannabinoids lower IOP through adrenergic receptors within the eye as well as through a mechanism involving prostaglandins by action of endocannabinoid metabolites,” wrote the study authors in their paper.

Characterizing the mechanisms behind cannabis’ effect on IOP, and any subsequent effect on glaucoma-affected eyes, should be a priority for future research, they say.

While medical cannabis might help to improve IOP, this does not mean it is necessarily an effective treatment for glaucoma. The American Academy of Ophthalmology (AAO) does not currently recommend cannabis products for the treatment of glaucoma, and contends that cannabis’ ability to lower blood pressure may present additional risk. In addition to high IOP levels, the optic nerve can also be damaged by low blood flow, and so it is possible that cannabis use could lower blood flow to the optic nerve, they say. As such, more study into cannabis formulations and their long-term effect on eyes with glaucoma is also needed.