There was a movie, I believe in the '90s, where there appeared an ancient worm-like creature with eyes and a big mouth. It does some convincing chat with a guy on a back alley, and has the man put the alien on the back of his neck like a parasite. The creature injects a needle to the man's spine or brain somewhere, with hallucinogenic liquid drugs (after the man does things, he asks as a reward of some sort).
I can't remember but at the end the man gets tired of this parasitic relationship and can't get it off of his neck, so he squeezes the alien while it's injecting its drug to the man's brain, causing a huge dose of it to be released, and the man's head explodes with rays of light coming out of it.
The alien, by the way (worm like creature), is not small but rather big and uncomfortable to have it latched on the back of the man's neck. Like, as big as an arm...
Can't find the movie; if someone can tell me the name, I'd appreciate it.