Cannabis use for glaucoma assists with decreasing intraocular pressure, slowing the progression of the condition. However, continual use of cannabis can lead to dependency of cannabis for glaucoma. Cannabis does not provide a long-term beneficial effect and results in addiction, dependency and increased number of risks.  Cannabis currently is not recommended due to the risks it poses not being worth the very brief symptom management. 

What is glaucoma?¹

Glaucoma is a progressive optic neurological condition, resulting in loss of sight. This is due to the loss of retinal ganglion cells and axons within the optic nerve. Due to the slow progression, glaucoma often remains hidden for long periods of time until there is severe optic nerve damage. This pattern of vision loss is unique to glaucoma compared to other vision loss conditions.

This eye condition is characterised by increased intraocular pressure (IOP) meaning increased pressure within the eye. This is a common association but a direct link between increased IOP and nerve damage has not been found. Even without this direct link, IOP is the target of many therapies to assist with glaucoma symptoms.

The primary diagnostic variants are:

• Primary – Arising without known cause
  • Open-angle
  • Angle-closure
• Secondary – Arising from injurious event and/or previous medical conditions
  • Open-angle
  • Angle-closure

Primary open-angle glaucoma (POAG) is the most common form of the condition and is the primary target of research. There are other more specific subtypes of glaucoma depending on the characteristics of the individual’s glaucoma. Most of the individuals diagnosed with glaucoma are over 40 years old, however, there are those younger than that which can attain glaucoma.

Currently there is no cure or treatments that are able to undo the nerve damage or vision loss. However, treatments are available to slow progression and damage of the disease. Laser treatments, incisional glaucoma surgeries and other medicinal therapies focus on decreasing IOP in both prevention efforts and minimising the impact of glaucoma.

Why use cannabis for glaucoma?^2,3

Cannabis use is a treatment researchers have been looking at to treat their IOP in glaucoma. Cannabis use has been observed to decrease IOP and help manage pain present in acute glaucoma. THC, the psychoactive component of cannabis, is found to be the desired drug for glaucoma treatment.

THC has been researched for its use for glaucoma symptoms for many years (dated back to 1971), and has been identified as the IOP-lowering component of cannabis. Studies have shown a medically significant decrease in IOP for 2-4 hours.

Cannabinoid receptors (CB1 and CB2) are present in many of the eye areas and optic nerve areas. These receptors induce cytokine signalling which triggers various cellular events. Researchers have not yet discovered the pathways associated with reduced IOP, however they hypothesise that CB1 receptor activation in various areas of the eye improve IOP through aqueous production and trabecular outflow. Other activating agents of CB1 were seen to have a similar effect to THC.

Synthetic forms of THC also have been hypothesised to have neuroprotective effects, improving optic nerve blood flow, and at low doses it did not present any psychoactive effects. The increase in blood perfusion is predicted to have beneficial effects for glaucoma and other eye conditions.

What is cannabis dependency and what are the risks?

Cannabis dependency^4,5

Cannabis dependence arises through tolerance. Tolerance to cannabis reduces the overall benefit of the treatment through the body becoming ‘used to it’. The body’s systems work against the drug’s effects and establish a new normal level of body function, restoring the body to what it previously was.

Tolerance development is common in various forms of drugs, especially recreational drugs, and reduces the overall efficacy with common use. This is true of cannabis.

An issue with tolerance and medical treatments is that to get the same desired effect you require a higher dosage of the drug. This comes with a variety of risks and costs, but the main issue is that when the body has adapted to this dosage you will require it again. This can build up dependency on cannabis.

Dependency means that due to the high levels of tolerance to the drug and the adaptations which have taken place, stopping the drug will actually cause a worsening effect. The body has assumed a drugged level is the norm, and without the drug the body's adaptations cause a misfiring of signals and produce a worse effect for a period of time. This can have temporary issues which can be hard to deal with, such as cannabis withdrawal syndrome, along with worsening glaucoma symptoms. So, to avoid these ill effects people continue to take cannabis for their glaucoma, resulting in a dependency on cannabis use.

Risks of cannabis use

There is evidence of positive effects of cannabis for IOP, but the risks outweigh the benefits of cannabis usage even beyond cannabis dependency.

There is a large risk for substance abuse and addiction to cannabis due to heavy daily use that would be required. The short duration of the beneficial effect means there is a constant need and a lack of true clinical efficacy. Users would have to take the dosage 6-8 times a day, which is not feasible in many circumstances.

 This is also the psychoactive component of cannabis and so has strong effects on the cognition of the person. The psychoactive effects of THC can leave the person:

• Lightheaded
• Elated
• Dizzy and disoriented

There are also physical effects of cannabis use:

• Nausea
• Constipation
• Mildly sleepy
• Fainting or loss of consciousness
• Orthostatic blood pressure drops
• Slight tachycardia
• Slightly elevated body temperature

With consistent use and dependence on cannabis these symptoms not only become more frequent but may also become more severe. Initially with low doses there should not be much of these adverse effects, but as dosage increases so do the side effects.³

While there is evidence of neuroprotective effects there is also evidence of neurotoxic effects. Retinal and ganglion cells are more likely to go under apoptosis due to altered neurotransmitter signalling. Loss of these neurons is what drives vision loss in glaucoma, so how useful is cannabis in the long-term?

Continual cannabis use has many other side effects which are concerning. This includes impaired immunity, cardiovascular and respiratory issues, psychiatric conditions, motor functions and more.

Should people use cannabis medicinally for glaucoma?

Simply put, the American Glaucoma Society recommended against marijuana use for glaucoma treatment in 2009 and this still holds true today, present within their news and position statements.

The research for cannabis use is lacking in efficacy and applicability. It has not been able to address the main issues with cannabis use in glaucoma. It is significantly better to use the other treatments available for the treatment of glaucoma.

What treatments are available?^4,6

There are many other treatments available for glaucoma. While none are able to cure the disease it can slow progression and help with symptoms. There are both medication and surgical interventions that assist with glaucoma management and IOP.

Topical drugs have been deemed the best choice:

• Prostaglandin analogs
• Beta-adrenergic blocking agents
• Alpha-2 adrenergic agonists
• Rho-kinase inhibitors
• Carbonic anhydrase inhibitors
• Adenosine receptor agonists

These drugs can be taken in monotherapy and fixed/non-fixed combination therapy. Each is an eye-drop (topical) which is taken as a routine. There are individual risks and side effects of each, but they are much better alternatives than cannabis.

The primary issue with these medications is adherence to the routine. Many patients find it difficult or don’t wish to adhere to constant medications for the rest of their lives. This is taxing and can be socially isolating, but it is the best course of treatment and is the most likely solution to help with glaucoma. Knowing what treatments require strict vs less strict adherence may be of use to patients.⁷

There are also surgical treatments which assist in IOP, especially when the previously mentioned drugs are not working. They are also considered if the patient is requiring 3 or more drugs.

• A trabeculoplasty is a laser surgical treatment that stimulates drainage outflow
• Trabeculectomy and glaucoma drainage devices both lower IOP by creating alternate drainage routes

Dietary supplements have also been suggested to help with neuroprotection. Many natural products found in food can assist in overall health, and this does apply to glaucoma too. Incorporating more of the desired products into your lives could be beneficial to a certain extent, keeping a healthy diet outside of the dietary supplements too.⁸

Summary

Glaucoma is a degenerative eye condition resulting in eventual loss of sight through loss of neuronal function in the optic nerve. Cannabis has been researched for 50 years now for its use in glaucoma and has been found to reduce intraocular pressure for up to 4 hours. However, this comes with risks of dependency on cannabis. Cannabis dependency can be bad for your overall health and for glaucoma, resulting in addiction, adverse side effects increasing, neurotoxic effects, and increased costs. Dependency will also reduce the efficacy of cannabis overall and can have major effects on a person’s abilities in their daily life.

There are much better, more effective and less adverse therapies available to be discussed. If adherence is kept there is a much better outcome for glaucoma progression. Cannabis is not recommended for glaucoma in any circumstance due to the ill effects it has and the better outcome presented by other treatments.