Introduction

Glaucoma is a chronic ocular condition that can result in irreversible blindness due to damage to the optic nerve. Intraocular pressure (IOP) is the only modifiable risk factor, making its management essential in the treatment of glaucoma. Research has shown that some cannabinoid compounds can improve blood flow to the optic nerve and lower IOP.^1,2 This article will explore specific cannabinoid compounds, THC (tetrahydrocannabinol) and CBD (cannabidiol) and their impact on an individual’s IOP.

THC and IOP

The cannabinoid compound THC has been shown to interact with vision-processing related receptors, specifically CB1 and CB2.^9,10 Through these interactions, THC can relax the ciliary body, widen Schlemm’s canal, and activate enzymes that clear blocked drainage pathways.^11,12 Aqueous humour is produced by the ciliary body, flows through the posterior chamber behind the iris, passes through the pupil into the anterior chamber, and drains via the trabecular meshwork and Schlemm’s canal to exit the eye.11,12,13 The circulation of aqueous humour helps maintain an individual’s IOP, which is key to managing glaucoma and minimising optic nerve damage.^1,2,11 

Furthermore, THC also increases the production of prostaglandins, natural chemicals in the body that encourage fluid outflow, further reducing IOP.^14,15 Additionally, studies using retinal imaging have shown that THC can improve blood flow in the retina, which enhances oxygen and nutrient delivery to the optic nerve.² This increased blood supply may help protect the optic nerve from damage, preserving vision in people with glaucoma.¹³

CBD and IOP

The cannabinoid compound CBD is known to be non-psychoactive, therefore there is a decreased risk of addiction for patient cohorts.^3,4 However, research on CBD and its effect on an individual’s IOP has been inconsistent; some authors have indicated that lower doses of CBD have no impact on an individual’s IOP, and others have shown that higher doses can cause a possible increase in an individual’s IOP.^16,17 

Additionally, some studies have conveyed that CBD can disrupt the IOP-lowering effects of THC.¹⁰ CBD is well-utilised in inflammatory conditions, such as rheumatoid arthritis, due to its anti-inflammatory properties. Therefore, further research is required to review any indirect impacts of this property on an individual’s eye.¹⁸ 

Clinical implications and limitations

While the cannabinoid compounds THC and CBD have shown some potential for their clinical uses, they also present significant challenges. Firstly, THC produces an euphoric "high" and can cause an individual to become addicted, thereby rendering it unsuitable for some patients.^3,4 

Secondly, although THC has been shown to be successful in reducing an individual’s IOP, the efficacy of pressure reduction is linked to the delivery method of the cannabinoid compound. Inhaled cannabis has been shown to be less effective at lowering IOP than topical administration.⁵ Although inhaled cannabis is effective in reducing IOP, it can have widespread effects on the body in addition to an individual’s eyes, Topical cannabis eye drops have been shown to be ineffective due to poor penetration through the cornea, preventing the necessary cannabinoid compound from reaching areas (such as the drainage angle of the eye) necessary for IOP reduction.^5,6 Therefore, additional research on the delivery method of cannabis is required to enable a localised effect on the body.⁵ 

Additionally, the IOP-lowering effects of THC are short-lived, which would require frequent doses in a day, thereby causing the possibility of addiction more likely for an individual.^6,7,8 Further studies are required to develop sustained-release effects of THC on IOP. Lastly, the exploration of combined cannabinoid compounds or even non-THC cannabinoids that lower IOP and do not have any psychoactive effects is required to enable safe administration. 

FAQs

What is the difference between CBD and THC?

CBD and THC are two primary compounds found in cannabis, but they have distinct effects and uses:

• CBD is a non-psychoactive cannabinoid compound; therefore, it does not cause a euphoric ‘high’ for an individual, ie there is a decreased risk of addiction. It can be used for its therapeutic effects, such as reducing inflammation and pain
• THC is a psychoactive cannabinoid compound, responsible for the euphoric ‘high’ and possible addiction for an individual. It has been shown to have successful effects on lowering IOP, but still requires further research

Is THC or CBD legal for glaucoma treatment?

The legality of cannabis compounds differs across the world. In many countries, medicines containing THC are restricted and usually require a medical prescription due to its psychoactive nature. Currently, THC is not medically advised as a treatment for glaucoma patients. However, CBD is more readily available and can be found in many pain relief medicines, especially as it does not have any psychoactive properties. CBD is not approved for managing glaucoma.

Summary 

In conclusion, THC has demonstrated success in lowering IOP in individuals, but the cannabinoid compound is limited by its short duration and psychoactive effects. CBD has shown a mixed review of its effect on IOP (in some cases increasing an individual’s IOP), but the cannabinoid compound is safer and more tolerated by individuals. Further research is required to explore cannabinoid compounds that don't cause psychoactive, have long long-lasting duration and are effective in reducing IOP for glaucoma patients.