Drew Barrymore failed to attend her first audition. Director, Katt Shea Ruben wanted Drew for the part of Ivy and arranged another audition which Drew Barrymore also failed to attend. After Katt Shea spoke to Drew's agent and told her she was no longer interested in her for the lead, she pleaded with Katt to give Drew another change. Drew ended up auditioning at Katt Shea Ruben's house not knowing that the director had already decided she had the job.
Sara Gilbert was called back after shooting had finished to do the very last voice over after Ivy had fallen to her death. The line "I still think about her, I guess, still love her. She might have been even more alone than I was" was thought up on the spot. The crew didn't like the line and one even said "What the fuck? Are you kidding me? She would never say she still loves Ivy" but director Katt Shea insisted that it's what the Character of Sylvia would have said.
Katt Shea Ruben came up with the title, 'Posion Ivy' after shooting began and instantly doubted it and thought it was silly. Only after the crew agreed with her and told her to 'go with her heart' did she stick with the title.
The trivia item below may give away important plot points.
The Character of Ivy was originally written to live at the end of the film. The script had Ivy leave the Coopers home at the end and go hitch-hiking. It was only when New Line Cinema told Director, Katt Shea, that they wanted Drew Barrymore's character to die, that she had to go back and film the ending once all the other shooting had finished.