This screens in the BFI's Cult strand and is also showing on Thursday 20th August. Details here.
Time Out review:
An intriguing premise: what if a certain species of LSD, a decade later, should begin to have an unexpected effect on its users' chromosomes? All over an American city, isolated individuals inexplicably slaughter their loved ones before going on the rampage. The film has a phenomenal opening, and makes the most of its plot possibilities, but the police's continual arrival at the scene of murder just in time to implicate the investigative hero will put a strain on any audience's credulity. Exploitation of a superior kind, nonetheless.
Here (and above) is the trailer.