I wrote a response to this yesterday morning and then accidentallly switched to another page and lost it. Hadn't had quite enough coffee yet!

Cannabis makes smokers' eyes bloodshot because once the THC and other cannabinoids kick in, it dilates (relaxes) peripheral blood vessels, which makes them more visible, especially when viewed against a white background. The non-cannbinoid particulate matter in the smoke can also irritate the eyes, and smoke can be drying, too, which further aggravates irritation.

They're not sure exactly why cannabis is good for patients with glaucoma, which is increased ocular pressure, but it probably has to do with the relaxation effect again. It's apparently a common misconception that cannabis dilates people's pupils. In tests, it does not. But a lot of people smoke in low or dimly lit rooms, which accounts for pupillary dilation. Cannabis does have an effect on people's visual tracking, causing slightly jerky side-to-side eye movements. That's probably from its effects on the cerebellum, which governs smooth, coordinated movement, or from the relaxation of the blood vessels in the lower, back part of the brain (the cerebellum).

Here are some various links I found that mention visual effects. Add these to your folder of info as you keep fighting the good fight!

Better night vision through cannabis | Science | Guardian Unlimited
Cannabis May Help Diabetic Eye Disorder » Netscape.com
http://www.fcda.org/pdf/physiology.pdf
Workshop on the Medical Utility of Marijuana
Cannabis .net : A Site for Stoned Eyes
Medical Marijuana: Glaucoma
birdgirl73 Reviewed by birdgirl73 on . eyes... From time to time i have these arguements with my mum about weed and all her arguements get crushed by me and my folder of cannabis activism info. But when she pulls out the- why does it make your eyes look funny, it must affect them somehow- arguement i am left defeated as i canot find anything to point out otherwise. Does anyone know anything about canabis and eyes, would really apreciate any info. (like even if she is right ) thanks. Rating: 5