First off, let me start by saying that I do not condone taking any illegal substances. Having said that, it would be ignorant of me not to acknowledge that cannabis has some great medical benefits, like pain management. In my opinion it is just a matter of time before it is available nationwide just like alcohol is today.
Ok, now that we have covered the pleasantries, let’s get to the question I get asked a lot by patients young, as well as the ones that are, young at heart, “Why do my eyes get supper red and feel really dry, every time I smoke?” 
First of all, eyes will get dry no matter what we are smoking, weather its cigarettes, hookah, or marijuana. Inhaling smoke of any kind tends to dry our nasal passages, which are connected to our eyes, so it makes perfect sense, why smoking of any kind, will dry out the surface of our eyes.
Let’s Get Scientific! 
The main ingredients in cannabis are Δ9-tetrahydrocannabinol (THC) and cannabidiol (CBD). They act on specific cannabinoid receptors, mainly cannabinoid receptors type 1 and 2, CB1, and CB2. Studies done on pigs have demonstrated that CB1 and CB2 receptors are present in porcine iris, choroid, lacrimal gland, and salivary glands. It’s also known, from rat studies, that when a CB1 or CB2 receptor is bound by one of the cannabis ingredients, it stops functioning as effectively, or gets downregulated.
So now to make sense of all of this mumbo-jumbo!
In a nutshell, when someone smokes cannabis, their eyes, and mouth become dry because the CB1 and CB2 receptors get locked by the marijuana ingredients and stop producing saliva and tears as it would under normal conditions. This leads to the dreaded dry mouth, and dry-red eyes many always ask about.
If you are a medical marijuana user, looking to help treat these side effects your medicine is causing, consult a dry eye specialist in your town or area.
Sources:
Schwitzer T, Schwan R, Angioi-Duprez K, Giersch A, Laprevote V. The Endocannabinoid System in the Retina: From Physiology to Practical and Therapeutic Applications. Neural Plasticity. 2016;2016:2916732. doi:10.1155/2016/2916732.
Callén L, Moreno E, Barroso-Chinea P, et al. Cannabinoid Receptors CB1 and CB2 Form Functional Heteromers in Brain. The Journal of Biological Chemistry. 2012;287(25):20851-20865. doi:10.1074/jbc.M111.335273.
