Ever since a 1971 study reported that smoking marijuana reduces eye pressure, the use of cannabis for glaucoma has remained a popular topic.
Indeed, it’s now commonly accepted knowledge that taking cannabis reduces eye pressure for several hours. More importantly, recent research highlights that the body’s endocannabinoid system is involved in eye health, and consequently, targeting it through cannabinoids and other compounds may be beneficial in glaucoma.
The Endocannabinoid System
First discovered in the 1990s, the endocannabinoid system is a network of receptors, endocannabinoids, and the enzymes that control the synthesis and breakdown of these compounds.
This system plays a critical role in human health through helping our bodies, maintaining homeostasis: a healthy state of balance. Research shows that the endocannabinoid system is involved in many key processes, such as cognitive function, immunity, pain, sleep, and even eye health.
So far, researchers have identified two main cannabinoid receptors: CB1 and CB2. Whereas CB1 is abundant in the central nervous system, CB2 is common in immune cells. These receptors respond to anandamide and 2AG-the two endocannabinoids made inside our bodies. In addition, they also interact with phytocannabinoids: cannabinoids that come from cannabis, such as THC and CBD.
The Endocannabinoid system and Glaucoma
All parts of the endocannabinoid system, including receptors, endocannabinoids, and enzymes, are present in ocular tissue, suggesting that they play a role in healthy eye function.
More importantly, there’s evidence that glaucoma affects endocannabinoid system function. For example, one study compared the levels of 2-AG, anandamide, and PEA, a compound closely related to anandamide, in normal and glaucomatous eyes.
Although AEA levels were similar, diseased eyes had lower levels of 2-AG and PEA in a part of the eye called the ciliary body, which plays an important role in controlling eye pressure. PEA was also lower in the choroid, one of the outer layers of the eye.
Meanwhile, a study in rats with ocular hypertension, a condition that causes eye damage similar to acute glaucoma in humans, had even more interesting findings. In this experiment, hypertension led to increased levels of FAAH, an enzyme that breaks down anandamide, as well as a decrease in CB1 receptors.
More importantly, when the researchers gave the rats a compound that blocks FAAH or one that resembles anandamide, the retinal damage was reduced. This suggests that glaucoma may reduce anandamide levels in the retina and that this reduction contributes to retinal cell damage.
Findings such as these have led researchers to propose that “it may be possible to improve glaucomatous outcomes through therapeutic modification of the endocannabinoid system.”
Research shows that the endocannabinoid system can be harnessed to help with glaucoma in two ways: reducing intraocular pressure and providing neuroprotection.
For example, a 1996 study showed that applying topical, anandamide-like compounds to the eyes of rabbits resulted in a significant reduction of intraocular pressure, confirming that the endocannabinoid system regulates this process. Similarly, numerous studies have shown that cannabis and THC produce the same effect.
In addition, recent studies report that the endocannabinoid system can help with glaucoma by protecting neuron cells from damage, an effect known as neuroprotection.
The main way that glaucoma leads to blindness is by causing the death of a specific type of neurons found in the eye: retinal ganglion cells (RGCs). These cells can continue to be lost even when people are taking eye pressure-lowering drugs.
As it turns out, the endocannabinoid system can protect these neurons from damage that occurs during glaucoma. For example, one study found that raising levels of anandamide by blocking the FAAH enzyme protected rat RGCs from this damage.
Taken together, current research shows that influencing the endocannabinoid system by increasing endocannabinoids or using phytocannabinoids can be beneficial to glaucoma sufferers. As such, medical cannabis preparations may be helpful.
Cannabis & Glaucoma
The first piece of evidence that medical marijuana can help glaucoma sufferers came in 1971 when researchers discovered that smoking cannabis resulted in a significant reduction of intraocular pressure.
As for the neuroprotective effects of cannabis, current research is limited to animal and cell culture studies. For example, one study examined whether THC or CBD can prevent neuron damage caused by glutamate, a compound that contributes to neuron death in glaucoma. The researchers found that both cannabinoids protected the neurons.
Although such findings are not as clear cut as the effects of cannabis on eye pressure, there is enough evidence that both endo and phytocannabinoids can provide neuroprotection to retinal cells damaged by glaucoma.
However, it’s important to note that all evidence points to THC as the main cannabinoid responsible for the pressure-lowering effects of cannabis. CBD, on the other hand, may actually increase eye pressure.
For example, a 2006 study found that while sublingual (under the tongue) administration of THC reduced intraocular pressure, CBD either had no effect or actually increased pressure at higher doses. Similarly, a recent study in mice found that topical application of CBD increased intraocular pressure.
However, this doesn’t mean CBD should be disregarded entirely since it still has neuroprotective effects. If anything, this gives more support to the practice of using whole-plant cannabis preparations that combine significant amounts of THC and CBD and all other active compounds found in the whole plant.
All in all, current research evidence suggests that medical marijuana can be beneficial to glaucoma sufferers not only through reducing eye pressure but also protecting neuron cells. However, its medical use has been limited by side effects.
Potential side effects of cannabis use
Although medical researchers remain optimistic about the potential of cannabis to help with glaucoma, eye doctors as a whole do not support its use due to adverse effects.
Most notably, cannabis is known to potentially cause psychoactive effects such as anxiety, paranoia, poor memory, and euphoria, collectively known as the cannabis “high.” Besides, cannabis has other adverse effects, including increased heart rate, palpitations, and reduced blood pressure. Crucially, lower blood pressure can decrease blood flow to the optic nerve, negating the benefits provided by reducing eye pressure.
These side effects are problematic because the pressure-lowering benefits of cannabis only last about 3–4 hours. Since eye pressure in glaucoma must be controlled at all times, this means people using cannabis would have to continuously suffer from these issues.
The significance of these side effects was highlighted by a study where nine people with glaucoma took THC capsules or smoked cannabis. All nine participants chose to discontinue the treatment after 1–9 months.
Current research evidence highlights that influencing the endocannabinoid system is a promising way to treat glaucoma. Studies show that we can use this system to not only reduce eye pressure but also provide neuroprotection.
This latter finding is especially significant because some people with glaucoma continue to experience vision loss even after their eye pressure is reduced. As such, there’s a need for glaucoma medicines that address both eye pressure and neuronal damage.
Although cannabis is one potential candidate for such medicine, its use is currently limited by its short duration of action and significant side effects. For this reason, eye physicians do not recommend cannabis for glaucoma.
However, that doesn’t rule cannabis out completely. Researchers are currently testing a wide variety of compounds that affect the endocannabinoid system, including phytocannabinoids, synthetic cannabinoids, and compounds that increase endocannabinoid activity.
It’s quite possible that a cannabinoid-based medicine with longer duration and fewer side effects may be developed in the future.
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