Introduction

Glaucoma is a common eye disorder. It impairs a nerve within the eye (optic nerve), and this damage can lead someone to completely lose sight.^1,2 Unfortunately, once vision is lost, it cannot be restored. Most often, it happens when the pressure inside the eyes (intraocular pressure) is too high. Glaucoma is so common that even findings suggest that about 60 million people all over the world are experiencing it. Even though an increased intraocular pressure above 21 mmHg is a key risk factor, some people develop glaucoma with normal pressure; for example, the Japanese population are at lower risk, and others with high intraocular pressure never get this condition. So, managing eye pressure is important, but it’s not the only factor to consider.^1,2

IOP is made by fluids inside the eye. When this pressure becomes too high, it can cause damage to the optic nerve and result in loss of peripheral vision. In other words, it affects one's ability to see things with the sides of your eyes. Gradually, if this is not treated properly, this will lead to the inability to see permanently.² 

Since the 1970s, cannabis has been carefully analysed and considered for its ability to lower eye pressure. The main ingredient in cannabis, that is tetrahydrocannabinol (THC), can reduce the intraocular pressure by attaching itself to the cannabinoid receptor 1 (CB1) that is present in the ciliary body of the eye.3,4 Cannabinol (CBN), which is a type of cannabinoid that does not intoxicate, has also demonstrated that it can lower eye pressure and protect nerve cells in the eye; these effects were based on studies done in the laboratory.⁵

Glaucoma and intraocular pressure

Glaucoma has a connection with intraocular pressure, and it is a gradual process. First, there is an increased level of eye pressure (IOP), which then causes damage to the optic nerve, which transfers vision information from the eyes to the brain, allowing us to see. When the optic nerve is damaged, it gradually affects the ability to see with the side of the eye, and subsequently, if it is not treated, it can lead to permanent loss of the ability to see. Therefore, controlling intraocular pressure is important in managing glaucoma because we have seen that a high intraocular pressure is a major risk factor for glaucoma and lowering the eye pressure is the only way that has been proven to either prevent or delay the progression of glaucoma.6 Glaucoma most often does not show symptoms until substantial damage is already done to eye vision; hence, it is important to have regular eye check-ups, especially for older adults or those at risk. The ophthalmologist set a safe pressure level as a baseline for each patient, depending on how damaged the eye is and other personal health factors.⁷ The traditional treatments for glaucoma, such as eye drops, laser therapy and surgery, focus on reducing Intraocular pressure, but there are existing arguments that cannabis, particularly the tetrahydrocannabinol (THC) component, reduces intraocular pressure.^7,8

Cannabis and its effects on intraocular pressure

There is a growing curiosity about the use of cannabis to treat glaucoma. One of the effects of cannabis on humans is its ability to lower eye pressure by close to 25%, although the effect only lasts 3 to 4 hours.₉ This implies that using cannabis to reduce IOP within a day, a person may be required to take it multiple times, which may not be practicable considering the potential effects cannabis has on the brain, heart and productivity._8,9

How does cannabis lower intraocular pressure?

Smoking cannabis reduced eye pressure from 17.5 to below 15 mmHg, which is similar to the regular glaucoma eye drops. The Tetrahydrocannabinol (THC) is a part of cannabis that causes one to be high (intoxicated), and also can reduce intraocular pressure, although for a short time. THC binds to a certain part of the eye, which is the cannabinoid receptor 1 (CB1) and then activates it. This causes the part of the eye that helps produce eye fluid that is the ciliary body, to reduce how much it makes, which helps the trabecular meshwork to drain out eye fluid better.^3,9

What are the common forms of using cannabis?

There are several ways people can use cannabis for intraocular pressure:^9,10

• Smoking cannabis: Works but may be harmful to the lungs
• Edibles like gummies: These are easy to dose but have a prolonged intoxicating effect
• Sublingual mints: Involves placing the mint under the tongue; it takes a short time to reduce intraocular pressure, but the effect does not last long
• Eye drops: This does not work well because the cornea does not absorb them properly. Another variant of cannabis, cannabidiol (CBD), is non-intoxicating and can be easily seen. Unfortunately, CBD, when tested in lab animals it was able to increase IOP to 18mmHg

Problems with using cannabis for glaucoma

Cannabis has the ability to reduce eye pressure (IOP), but only lasts for a few hours; hence, to maintain a low IOP for a day, it may need to be taken more than three times a day, which is not safe considering its side effects.⁹

Tolerance to cannabis may develop subsequently, as over time the body gets used to the dose of cannabis taken regularly and may need a higher dose to achieve a lower IOP, causing more side effects.¹⁰

Cannabis causes several unpleasant effects that affect one's productivity over time, which are not limited to intoxication (feeling high) and hypotension (low blood pressure).¹⁰

Side effects of cannabis on vision

Cannabis use can disrupt the normal process in human sight, and this can be for a short or long period of time.

Short-term side effects of cannabis on vision

Blurry vision: cannabis can make it harder to see things clearly, especially things closer to you, for example, when you are reading. This is because it affects the eye's ability to focus properly and makes it harder to see contrasts (differences between colours or brightness).¹¹

Changes in Pupil Size: Cannabis actually makes the pupils slightly smaller and slows down how fast they adjust to light. This can make it harder for your eyes to adapt to changes in light entering the eye, like when going from a bright room to a dark one.^11,12

Poor Depth Perception: Cannabis interferes with a person’s ability to see in three dimensions, making it harder to judge distances. This can increase the risk of accidents, especially while driving or moving around.^11,13

Light Sensitivity and Night Vision Issues: It increases glare (lights are too bright and uncomfortable to look at) and causes halos (growing circles) around lights, making it uncomfortable to look at bright lights. It also worsens night vision, doubling the amount of “visual disturbance” people report after using it.¹¹

Long-term side effects of cannabis on the eye

Optic Nerve Problems: A recent study recorded serious damage to the eye nerve that transmits vision information to the brain 8 hours after smoking cannabis. Long-term use might also affect how the eye sends signals to the brain, causing delays in interpreting visual signals.¹⁴

Risk of Dependence: With cannabis, there is are increased chance of users becoming dependent. This is a problem if used in glaucoma treatment because, over time, the eye gets used to THC, so users will need more cannabis to get the same effect that the initial dose gave, which also increases side effects.^11,15

Reduce blood pressure: Cannabis causes a drop in blood pressure, which has a negative effect on the eye because the eye needs steady blood flow and if the blood pressure is too low enough blood won't get to the eye, thereby reducing the amount of oxygen and nutrients that get to the eye nerves. This eventually worsens the condition of the eyes, including glaucoma.¹⁶

Dry eyes and irritation: Cannabis causes dryness, which reduces the quantity of tears produced. This consequently leads to redness, itching and burning of the eyes.¹⁷

What doctors and scientists say about using cannabis for glaucoma

While cannabis can temporarily reduce intraocular pressure, which is crucial in glaucoma treatment due to the THC that is contained in it, doctors and scientists have shared their concerns about its use in glaucoma therapy. They have warned against the use of cannabis instead of the regular glaucoma treatment. Their arguments are based on the following:¹⁸

• Glaucoma treatments need 24-hour intraocular pressure control, which can not be provided by cannabis. To effectively benefit the the lowering intraocular pressure effect of cannabis, one may need to use it several times in a day, which can cause unfavourable side effects like feeling high, dizziness and trouble focusing
• Cannabis is associated with low blood pressure, which eventually affects the eye and worsens glaucoma by reducing blood flow to the eye
• There is no standard dosing unit for cannabis, which, depending on how often it is used, may lead to complications like a very fast heartbeat and eye irritation

Hence, eye doctors (ophthalmologists) through the American Academy of Ophthalmology and the American Glaucoma Society have concluded that cannabis is not safe or effective for treating glaucoma due to the deleterious effects it has on eye health and further worsens vision. Therefore, they have recommended that individuals stick with proven treatments such as prescription eye drops, laser therapies and surgeries.¹⁹

Conclusion

In summary, while cannabis can provide lower eye pressure (IOP) for a short period of time, it causes effects that may harm our vision by affecting the ability to see properly and clearly. Due to the associated side effects linked to cannabis, ophthalmologists have warned against its use for glaucoma therapy.