It is the THC in marijuana that causes the red (bloodshot) eyes. THC lowers the blood pressure. When the blood pressure is lowered, blood vessels dilate and more blood is pushed through the body. Decreased blood pressure and more blood flow cause the eyes to get a little bigger (like expansion in the roads when it is hot out) because of the arteries moving so much blood at once. This is what causes the eyes to become bloodshot. This occurs when THC is taken in any form, including ingesting.
I know you don’t think the smoke can cause the red eyes but it can in certain circumstances, like if the wind changes toward someone at a bonfire. Pot heads (heavy marijuana smokers) like to do something called “clam-baking.” Clam-baking is the heavy smoking of marijuana in a smaller area which has really no air flow. It is believed that this will allow the smokers to get even more “high” because of the smoke continuously being breathed in even while not physically smoking. When in a confined area with poor air circulation any smoke, including weed, can cause an irritation in the eyes causing them to become red.
Some people are allergic to marijuana. Cannabis (the marijuana plant) is just that – a plant. In order to reproduce, plants need to make seeds. This is done by pollination. Many people are allergic to pollen. This will cause more than just bloodshot eyes, such as hives and even passing out.
Some people are predestined to get bloodshot eyes when smoking marijuana. Sometimes the bloodshot eyes can be for different reasons when smoking marijuana such as:
- Genes: People have different genes of tolerance for anything and everything. Reactions to plants, including marijuana, are no different. Some people just have an intolerance.
- Frequent Use vs Infrequent Use: Some people build up a tolerance to marijuana and create a resistance to how it affects them, including their eyes. Infrequent, or new users, usually have more of an issue with bloodshot eyes.
- Potency: Some strains of cannabis are stronger than others. Crossbreeds, and higher generations, of marijuana can (and usually does) make a person have more of a reaction (from “highs” to physical reactions such as bloodshot eyes). These strains are generally created to be stronger.
For more effects of marijuana on your body, visit: http://www.healthline.com/health/addiction/marijuana/effect-on-body