Red Eyes are almost always associated with marijuana usage. But why exactly do your eyes turn red, after you just hit the bong? Well there are many factors that contribute to this physical change. According to The New Health Advisor, "The most common reason for eye redness is the THC compound in weed. THC can act upon the body and the eyes in particular regardless of the form of ingestion. It works by decreasing the blood pressure thereby dilating the blood vessels of the body. Dilated blood vessels lead to increased blood flow around the body. Under low blood pressure, the arteries in the eyeball expand, get filled with blood, and thus cause the redness."
So in short, THC is the compound in marijuana that causes you to feel "high", this compound decreases your blood pressure and that in turn causes the arteries in your eye to expand and fill with blood. But, it should also be noted that as you begin to smoke more regularly, you might develop resistance to red-ness in your eyes. The reason that your eyes may turn red on some strains and not on others is because of the amount of THC. You may smoke a high THC strain one day and have super red-eyes and a low THC strain on the next and have no red-eyes at all. There are many variations to why one could have red-eyes after smoking.
These aren't the only reasons your eyes might turn red after smoking. Some people have experienced eye irritation to weed smoke, and because of this irritation it causes a red-ness to appear in the eye. Another reason could be an allergic reaction to marijuana. Although, the chances of being allergic to marijuana are very slim, it is still possible and could be the reason to why your eyes are red after smoking.