A sea change is occurring in the off-patent drug manufacturing industry with a first wave of biotechnologically derived products reaching the end of their patent lives. However, recombinant proteins are in a different league from their chemical predecessors in terms of molecular complexity. Small differences in manufacturing processes can affect the efficacy and safety of the recombinant proteins in a manner which is not always measurable using analytical or in vitro techniques. Thus, comparable clinical profiles do not automatically follow from physicochemical likeness and can only be demonstrated through clinical studies. It is essential for patient safety that both innovator and biosimilar manufacturers ensure consistency in their production, by performing rigorous purity and activity profiling between batches, and implement tailored pharmacovigilance plans.