My main problem with this movie wasn't that there are aliens. Nor that these aliens were after heroin or human brains. You just have all these dysfunctional relationships between the characters and nobody ever goes postal. Even talking to most of these people would be like getting beaten in the head with a sock full of quarters. Aliens are best represented by something which belongs in an eyedrop ad.
Microphones make handy stethoscopes.
Philosophy is less convincing when dispensed by a heroin addict. Telescopes are equipped with crosshairs. Some women take pride in being told they dress like a hooker. Junkies often mistake German scientists for police officers. The brain produces a chemical that tastes just like heroin during orgasm.