After smoking marijuana my eyes also get so red. As for my other friends eyes dont get red or that red. I know their getting high though. Also why do some people get cottinmouth and some people dont? Doesnt that have to do something with nationality?
its from an irritated rectum. try cleaning yourself out with a hotsauce enema, it helps 80% of users. your bloodvessels in your eyes get contricted/expanded and your eyes get that color. wash your eyes out with water before and after you smoke and it will help a bit. if you have high blood pressure, your eyes get red easier.
Both red eyes and cottonmouth are partially linked to the fact that the part of the brain that governs salivation and tear production is affected by cannabis in some/many people. The other component of red eyes is the fact that THC is a vasodialotor; i.e. it causes blood vessels to expand slightly. Incidentally this is also one of the reasons that cannabis smoke is not quite as harmful as tobacco smoke. Nicotine is a powerful vasoconstrictor, and hence causes blood vessels to contract in lungs and elsewhere (including the heart).