Peter Grimmer
Written by: Peter Grimmer
Wellington Eye Centre Optometrist

Immediately after smoking marijuana or ingesting marijuana edibles, the eye becomes red due to dilated conjunctival blood vessels. But you may wonder what other effects does marijuana have on the eyes?

But ever since scientists recorded a lowering of intra-ocular pressure with marijuana, hopes for integrating it legally into the armoury of treatments for primary open-angle glaucoma have been – well – high.

Primary open-angle glaucoma is a type of glaucoma, defined by;

  • normal appearing anterior chamber (front of the eye anatomy)
  • raised intraocular pressure (IOP)
  • damage to the optic nerve head
  • vision loss
  • but with no underlying disease

In New Zealand it is currently illegal to be in possession of, cultivate or supply cannabis unless you have a prescription from a medical professional for medicinal cannabis.

The cannabis plant contains about 540 natural compounds, more than 100 of which are classified as cannabinoids. Two of these cannabinoids are; D9-tetrahydrocannabinol (THC) (recognised as the primary psychotropic compound) and cannabidiol (the predominant non-psychoactive ingredient).

Studies have shown that THC may result in a reduction in Intraocular pressure (IOP).

But I’m afraid the literature so far is not promising.

THC delivery intravenously (by injection into a vein) resulted in a 15-20% reduction in IOP, however, the effect only lasted for 3-4 hours.

Oral (by mouth) THC failed to demonstrate a significant lowering of IOP.

Delivery by topical drops (eye drops) also failed to demonstrate a lowering in IOP and in fact these eye drops were very irritating to the eye.

And the one you are waiting for – inhalational (smoking weed folks) resulted in a reduction in IOP by 25%. But – it only lasted 2-4 hours and had completely worn off by 4 hours.

Effectively you would then need to smoke marijuana continuously to maintain a lowering of IOP and with that comes a myriad of undesirable short-term side effects. Side effects include heart palpitations, being too cognitively impaired to operate machinery or generally function properly. Even driving a car would be problematic, so that approach hit a few potholes.

Smoking marijuana also has long-term effects such as lung disease, lowered blood pressure, cognitive disability, and mental health issues, all posing a significant problem.

Frankly, right now, any thought that marijuana is going to help in the treatment of a significant eye disease like glaucoma is – well – wasted.

What about Laser Eye Surgery and marijuana?

So, tongue in cheek aside, what are the pragmatics around laser eye surgery and – your use of marijuana?

Well, firstly we are concerned about decreased corneal sensation caused by marijuana inhalation or ingestion.

Decreased corneal sensation leads to a decreased blink rate and with that dry eye issues. Since laser eye surgery can already cause dry eye issues, you don’t need another factor contributing to that.  

There is a remote possibility that the loss of corneal sensation will mean you could be unaware of complications post-surgery.

Marijuana smoke, just like cigarette smoke, can be an eye irritant. With all smoking there is an increase in sputum production,  meaning you may be more likely to cough during the surgical procedure.

As a general and non-specific rule, we feel you should abstain from ingesting or inhaling marijuana for a week pre and post-surgery.

We wouldn’t want you to confuse or associate your new vision and perspective on life to be attributed to THC rather than the expertise delivered here at the Wellington Eye Centre – now that was tongue-in-cheek!

If you have any questions get in touch, we’d love to help!

If you’d like to learn about laser eye surgery then check out our other blogs about LASIK or SMILE surgeries. You can also book a free assessment or call us on 0800 733 327 for more information.

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