THC reduced eye pressure by 16%, but only up to a plasma threshold | Research

The takeaway

In 11 healthy adults, inhaled cannabis reduced intraocular pressure by an average of 16% at 60 minutes, with a strong correlation to plasma THC levels up to 20 ng/ml but no further reduction beyond that threshold.

Ophthalmologists, glaucoma patients, and anyone interested in the dose-response relationship between THC and eye pressure.

16% IOP reduction at peak; ceiling effect at plasma THC 20 ng/ml

What the researchers found

Peak IOP reduction was 16% at 60 minutes post-inhalation. THC plasma levels correlated strongly with IOP reduction (r = -0.81 for percent reduction) up to 20 ng/ml. Above 20 ng/ml, the correlation disappeared (r = 0.21), suggesting a ceiling effect.

Why it matters

Cannabis has long been discussed as a potential glaucoma treatment, but understanding the dose-response relationship is essential. The finding of a ceiling effect at 20 ng/ml plasma THC suggests that higher doses would not provide additional IOP benefit.

The numbers in context

11 subjects, 22 eyes; peak IOP reduction 16% at 60 min; correlation r = -0.81 below 20 ng/ml; r = 0.21 above 20 ng/ml; change point statistically significant (P < 0.01)

How the study worked

Eleven healthy subjects self-administered a single dose of inhaled cannabis. IOP and plasma THC were measured at baseline, every 30 minutes for the first hour, then hourly for 4 hours. Linear regression and two-line regression models with F-tests identified a change point in the THC-IOP relationship.

What this study cannot tell us

Very small sample (11 subjects). Healthy adults only, not glaucoma patients. Single-dose study. Short follow-up of 4 hours. IOP management requires 24-hour control.

How to read the evidence

Very small pilot study in healthy volunteers with careful pharmacokinetic-pharmacodynamic correlation, but no clinical disease model.

When this study was published

Published in 2021.

The bigger picture

The ceiling effect at 20 ng/ml challenges the idea that more THC means more eye pressure reduction. This pharmacological detail is important for anyone considering cannabis-based approaches to IOP management.

Questions still open

Does the ceiling effect persist with chronic use? Would the IOP response differ in glaucoma patients? Can targeted THC dosing achieve sustained 20 ng/ml plasma levels without psychoactive side effects?

Common questions

Can cannabis treat glaucoma?

THC does reduce eye pressure, but this study shows the effect plateaus at moderate blood levels. The 4-hour duration is also too short for 24-hour IOP control that glaucoma requires.

Does more THC mean more eye pressure reduction?

Only up to a point. Above plasma levels of 20 ng/ml, additional THC did not produce further IOP reduction in this study.

Read the original research

The Relationship Between Plasma Tetrahydrocannabinol Levels and Intraocular Pressure in Healthy Adult Subjects.

Frontiers in medicine, 8, 736792

View on PubMed ↗Publisher’s paper ↗

Citation

Mosaed, Sameh; Smith, Andrew K; Liu, John H K; Minckler, Donald S; Fitzgerald, Robert L; Grelotti, David; Sones, Emily; Weinreb, Robert N; Marcotte, Thomas D. (2021). The Relationship Between Plasma Tetrahydrocannabinol Levels and Intraocular Pressure in Healthy Adult Subjects.. Frontiers in medicine, 8, 736792. https://doi.org/10.3389/fmed.2021.736792