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Biosimilars, or similar biological medicinal products, are large, complex molecules manufactured using living cells. Unlike their small-molecule counterparts, identical copies of the originator product are nearly impossible to manufacture. Biosimilars require an extensive manufacturing process that is open to variation at a number of stages and as such requires very careful regulation and monitoring. With the patents on a number of biologically engineered proteins nearing their end, increased interest has arisen regarding how the development of biosimilars should be regulated. The EU is currently adopting guidelines that address a number of issues surrounding the generation of biosimilars, including the nonclinical and clinical aspects of comparing the biosimilar with a reference product. However, questions such as the choice of the reference product, the level of postmarketing surveillance and even the name of the biosimilar, all have still to be addressed. In contrast, the US has yet to agree on guidelines for the introduction of biosimilars and it would appear that these are still a little way off. While some believe that adequate measures are already in place to safeguard against the introduction of biosimilars with unproven safety and efficacy profiles, previous experience would suggest otherwise. Data from various interferon preparations have shown that the immunogenic potential, i.e. the ability of the product to induce the production of neutralising antibodies, varies according to formulation and place of manufacture. In addition, the increased incidence of pure red cell aplasia with a certain preparation of epoetin alfa after the formulation was changed has demonstrated that relatively small changes in the preparation of even established biopharmaceuticals can have serious implications for the patient. Therefore, it is vital that regulatory guidelines, as much as is reasonably possible, ensure that biosimilars will be safe and effective in clinical practice.