Rubber components are essential for the safe containment and delivery of parenteral drug products. Many rubber formulations used in the pharmaceutical industry are based on halobutyl rubbers. Halobutyl rubbers, typically bromobutyl or chlorobutyl, are made by the halogenation of butyl rubber, which is a copolymer of isobutylene and isoprene (to a smaller extent). Key features of halobutyl rubbers that make them suitable for the pharmaceutical industry are that they typically have lower levels of extractables compared to other rubbers, and they have excellent resistance to permeation by water and oxygen.