system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How come doctors have had such trouble treating diseases like Alzheimer's because of the impenetrability of the blood brain barrier but chemicals like marijuana and alcohol have no trouble at all?
gpt
The blood-brain barrier has nothing to do with why we can't treat Alzheimer Disease. There are no medications available to directly combat it.

It does act as a filter for large molecules and cells (such as bacteria). It is not just a "net" with tiny holes it though, it's more complex than that.

The blood-brain barrier allows lipophilic molecules to pass through (with some exceptions that are actively transported out by specialized proteins). Being lipophilic means it has certain chemical properties, but the basic thing is that is able to dissolve in fat/oil (aka lipids). Alcohol and THC dissolve in oil and are relatively small molecules, therefore they pass right through.