Biosimilars vs generics: what is the actual difference?

A small-molecule drug such as atorvastatin is a defined chemical structure that can be synthesised to be identical every time. A generic is therefore chemically the same molecule.

A biologic such as trastuzumab or adalimumab is a large protein produced by living cells. Even the originator's own batches vary slightly within accepted limits, because biological systems are not chemical reactors. An exact copy is not scientifically achievable.

Hence 'biosimilar' — highly similar, with no clinically meaningful differences — rather than 'generic'.

Approval rests on a comparability exercise rather than a single study: extensive analytical characterisation of structure and function, non-clinical work, pharmacokinetic comparison, and clinical data in a sensitive indication where a difference would be detectable.

Regulators may then permit extrapolation to other indications of the reference product where the mechanism of action justifies it. Post-marketing safety monitoring, including immunogenicity surveillance, continues after approval.

If you are offered a biosimilar, it is reasonable to accept it when your specialist recommends it — this is now standard practice across many oncology, rheumatology and gastroenterology services.

Practical points worth raising: record the specific product name and batch number at each administration, since biologic pharmacovigilance is product-specific; ask whether the delivery device differs, as device technique matters; and agree what will be monitored after the switch.

• Note the exact brand and batch at every dose.
• Ask whether the injection device or administration method changes.
• Agree monitoring parameters and a review date.
• Report any new injection-site or systemic reaction promptly.