This is an unusual case: a treatment with a real effect that eye doctors still advise against. Cannabis really does lower pressure inside the eye, but only for about three to four hours. Glaucoma damages the optic nerve twenty-four hours a day. Work out what that means for dosing and you reach the ophthalmologists’ position without anyone telling you.
Nothing here is medical advice. Glaucoma is a leading cause of irreversible blindness, and the vision it takes does not come back. If you have glaucoma, talk to your ophthalmologist about this. In Thailand cannabis flower is a controlled herb dispensed only on prescription form PT 33.
1. The Finding
Glaucoma is a group of optic neuropathies. Raised intraocular pressure (IOP) is the main risk factor that can be changed, and lowering it is the only treatment proven to slow progression. Glaucoma can also occur at normal pressure.
Researchers first observed in the 1970s that cannabis lowers IOP, and nobody disputes it. The first report (Hepler and Frank, 1971) came from people smoking cannabis. Later studies in glaucoma patients, such as Merritt et al. (1980), measured falls in both intraocular pressure and blood pressure.
The size of the effect is usually reported as follows: smoked cannabis lowers intraocular pressure in about 60–65% of users, by roughly 25% (Green, 1998; Daga et al., 2021).
So the mechanism is not in doubt and the data is not thin. The drug does the thing.
2. Why That Is Not Enough
The effect lasts three to four hours.
Glaucoma is a continuous process. Optic nerve damage does not pause overnight, and pressure control that lapses for most of the day is not pressure control. That gives you the arithmetic that settles the question. Continuous 24-hour control would take roughly six to ten doses a day (one every 3–4 hours), every day, indefinitely (AAO, 2014; Daga et al., 2021).
Nobody can function on that schedule. A patient who tried would spend their entire waking life intoxicated to treat a condition they cannot feel. That is the second reason this fails: early open-angle glaucoma usually causes no symptoms, so there is nothing to feel better from.
3. The Part That Turns It From Impractical to Potentially Harmful
There is also an objection at the level of mechanism. It is more serious than the dosing problem and gets less attention than it should.
Cannabis causes tachycardia and decreased blood pressure, and lower systemic blood pressure can reduce blood flow to the optic nerve. The American Academy of Ophthalmology has warned that this could cancel out the benefit of a lower IOP.
In glaucoma, the optic nerve is already compromised. Perfusion pressure, meaning the blood supply reaching the nerve head, is thought to play a part in how glaucoma progresses. So a drug that lowers eye pressure while also lowering the blood pressure that feeds the damaged nerve is working against itself. Its net effect on what actually matters, preserved vision, has not been established.
That is the crux. Cannabis studies in glaucoma have measured intraocular pressure. No published controlled trial has shown that cannabis preserves visual field or slows optic nerve damage. IOP is a surrogate endpoint, and this is a case where the surrogate and the real outcome could plausibly diverge.
Two practical problems make it worse. Tolerance develops with chronic use, so the effect weakens over time. And potency varies widely between products, so the dose delivered is unpredictable.
4. The Professional Position
The American Glaucoma Society, in its 2009 position statement (published in the Journal of Glaucoma in 2010), acknowledges that marijuana lowers intraocular pressure. It concludes that the side effects, the short duration of action and the lack of evidence that it changes the course of the disease rule out recommending it in any form for glaucoma at the present time.
The American Academy of Ophthalmology restated its position in 2014: marijuana is not a proven treatment for glaucoma, and the Academy does not recommend marijuana or other cannabis products to treat it.
Note the shape of the Glaucoma Society’s statement. It does not say “the evidence is weak” or “we do not believe it lowers pressure.” It accepts the effect and rejects the treatment for specific reasons: duration, side effects and no shown benefit to vision, all “at the present time.” Solve those problems in rigorous trials and the reasoning changes. That is a scientific position, not a political one, and it is the right way to frame this whole topic.
5. The Point That Generalises
This case is unusual. It shows a practical clinical limitation, not weak evidence, and that distinction is rare enough to spell out.
Most cannabis health claims fail because the evidence is thin: preclinical only, uncontrolled, or too small. This one fails despite good evidence, because of pharmacokinetics and route of delivery.
Three properties decide whether a real effect becomes a usable treatment, and cannabis fails all three here:
| Requirement | Cannabis in glaucoma |
|---|---|
| Duration matched to the disease | 3–4 hours against a 24-hour disease |
| Delivery compatible with a life | Roughly 6–10 doses daily, with ongoing intoxication |
| Side effects that do not undo the benefit | Lowered blood pressure may reduce optic nerve perfusion |
Existing glaucoma treatments are a different picture. Topical prostaglandin analogues, beta blockers and others are applied to the eye once or twice daily, and laser and surgery are also options. They do not intoxicate, and they have decades of outcome data on preserved vision, not just on pressure.
6. What Would Change This
Research into the underlying pharmacology continues, and there is a legitimate direction. That is a topical formulation that reaches the eye without systemic exposure, which would remove both the intoxication and the blood-pressure problem, and that lasts long enough to be dosed once or twice daily. Formulating cannabinoids for the eye is genuinely difficult: they are highly lipophilic and the eye is an awkward target. But this is the version of the idea worth pursuing.
If it works, it will be a cannabinoid eye drop with a trial behind it, prescribed by an ophthalmologist. It will not be cannabis.
7. In Thailand
Cannabis flower is a controlled herb under the Notification of the Ministry of Public Health Re: Controlled Herbs (Cannabis), B.E. 2568, in force since 26 June 2025. It is dispensed on prescription form PT 33, and a prescription covers at most 30 days. Glaucoma is not an established indication for medical cannabis. Given the perfusion concern above, a prescriber who knows about a glaucoma diagnosis has good reason to see it as a reason for caution, not a reason to prescribe.
The practical advice for anyone with glaucoma is unglamorous and important: use your eye drops. Poor adherence to eye drops is one of the most common reasons glaucoma progresses. Often the vision is lost not because the treatment failed but because the drops were not used.
If you have glaucoma and have asked an ophthalmologist about cannabis, what reason did they give? We would expect the duration problem. If anyone was told about optic nerve perfusion, please record it here: it is the more serious objection and the one mentioned less often.
References
- Jampel H. American Glaucoma Society position statement: marijuana and the treatment of glaucoma. J Glaucoma. 2010;19(2):75–76. (Statement issued 2009.)
- American Academy of Ophthalmology. American Academy of Ophthalmology Reiterates Position that Marijuana is Not Proven Treatment for Glaucoma. News release, 2014. https://aao.org/newsroom/news-releases/detail/american-academy-of-ophthalmology-reiterates-posit
- Green K. Marijuana smoking vs cannabinoids for glaucoma therapy. Arch Ophthalmol. 1998;116(11):1433–1437. doi:10.1001/archopht.116.11.1433
- Merritt JC, Crawford WJ, Alexander PC, Anduze AL, Gelbart SS. Effect of marihuana on intraocular and blood pressure in glaucoma. Ophthalmology. 1980;87(3):222–228. doi:10.1016/s0161-6420(80)35258-5
- Daga FB, Almeida I, Prata TS, Paranhos A Jr. Marijuana for glaucoma treatment: a recipe for disaster. Arq Bras Oftalmol. 2021;84(2):V–VII. doi:10.5935/0004-2749.20210056
This article is general educational information on the science of medical cannabis, not medical advice. Glaucoma causes irreversible vision loss; do not substitute anything for prescribed treatment.