The first time I tried cannabis, I freaked out because my eyes went bright red and I didn’t know why. Later, I read https://unitedrecoveryproject.com/rehab-blog/high-eyes/ which explained that this happens because THC dilates blood vessels in the eyes. It made me feel a lot better knowing it’s normal and harmless, and I also learned a few tips to minimize the redness. It’s such a small thing, but understanding it made the experience much less stressful.