Shit happens, but some folks are just begging for it. I'm sure a normal dose of, say, L, couldn't do that to a normal guy.
Depends. Most drugs affect the application layer of what makes you.... you. They make small changes that go away when rebooted. Pot and alcohol fall in this category.
A few affect the operating system that the thing called "you" runs on. They make more substantive changes in how you act and change a number of things that make you you. Good opiates fall in this category.
LSD and such twiddle around with your firmware or in some cases your microcode. They can change who you *are*, and can rewrite the thing your mental operating system runs on. Use with care.