Six ounces of robotussin.
What I had wasn't really quite a hallucination, it was more of a mindfuck, if anything... but I saw some of it, and I imagined the rest in my head. I was sitting in a pitch black room with two other friends. They were on a bed, on their backs, watching the lightshows behind their eyelids and I sat in a chair adjacent from the walk-in closet. It was darker in there than it was in the room and while Amnesiac (that Radiohead album) played in the background, I just started staring blankly into the closet. Out comes a little girl, white cotton night shirt down to her ankles, black hair down to the small of her back (I couldn't see it, but I just knew it reached the small of her back for some reason), and pale as a sheet. I'd say she was about five or six years old in appearance. She walked up to me cautiously, stopped about three feet in front of me, looked me up and down as if sizing me up, turned, walked out of the room, down the hallway, and dissipated when she reached the end. I'm sitting there, head full of DXM, floating a few feet above myself, going, "Alright... I think I've finally gone nuts." I, being the dumbass on drugs that I am, went back to staring at the closet. Another little girl comes out, identical to the first, and does the exact same thing. Looks me up and down, walks down the hall, vanishes. By the fourth girl, I yell, "Holy fuck, there are midgets everywhere!" and one of my friends on the bed goes, "Shut the fuck up, I'm trying to stay focused here! Just kick 'em or something!" or something to that extent. So I make this half-assed motion to punch and kick at the same time and almost fall out of my chair. They stop coming. I, once again being a dumbass, go back to staring into the closet. Another little girl comes out, this one's number five, and she looks like like the ones before her. She doesn't make it to me, though... she stops halfway and bursts into flames. In my mind, I can see the flesh melting off of her skeleton and disappearing before it hits the ground. After a couple of moments, the only thing in front of me is a bleached-white skeleton with a ruby the size of my fist floating in the ribcage. A moment later, the vision crumbles into dust.
Let me expel any weird myths that might be started by a post like this: I did not SEE all of this, a lot of it was actually just happening in my head. In all actuality, I'm pretty sure I just saw the outline of a small body rather than the whole girl and everything else was just added on.
Same night, I was walking down the hallway where the little girls kept disappearing, but I was walking back to the room. My friend's roommate's room was open and there was a streetlight right outside his window. The light was coming into the hallway in rays and it was filtering through dust motes hanging in the air. These dust motes took shape and morphed into the vague form of a gorilla walking in front of me on all fours wearing.. get this... a collared shirt. I swiped my hand through the dust motes and the gorilla disappeared.
Cannabis
I've only hallucinated once on cannabis, and even then for only a second. We were all standing outside of a friend's house after smoking some decent, high-grade stuff out of a gravity bong. I look down at my right arm and its covered in ants, so I go, "Holy fuck!" and rip off my shirt. Everyone's just kinda giving me sidelong glances... when I look back at my arm, there's nothing there. I was convinced that there had been ants on my arm for the entire night.