system
Given the following post, respond as if you're commenting on Reddit:
human
[GTAV] Why did Michael have vivid hallucinations of aliens but Franklin was still stone sober during the side mission where they interact with that 'activist'?
gpt
Each main character is a lot of things. But they all also have different backgrounds if you remember correctly.

Franklin is on medical marijuana, and interacts with it regularly. Trevor, on the other hand, huffs gas and does Meth. Michael has a history of tobacco and alcohol, but those are fairly tame. The point is that Michael and Trevor have little experience with weed, and thus mostly got blind sided by the special strain Barry has. Trevor even says that he would rather suck d*ck than smoke marijuana, and he knows because he has done both. Michael mentions a preference against his son's weed habit not because of the unhealthy lifestyle (he is a hell of a lot worse, after all) but because he dislikes weed itself.

So, in short Franklin isn't special, he was just the only one of the three who actually has experience with odd, special, and otherwise atypical strains of weed. And experiments have shown that you can develop smoking weed similar to a skill, with limits of course.