Most drugs are small molecules, synthesized in a flask, weighing a few hundred daltons. Biologics are different. They are large, complex proteins or nucleic acids produced by living cells, and their structures can be thousands of times bigger than aspirin. Insulin, monoclonal antibodies, and mRNA vaccines all belong to this category.

Making a biologic is not like making a chemical. You grow the cells that produce it, then harvest and purify the protein. Small changes in temperature, pH, or the cell line can alter the final product in ways that are hard to predict. That complexity is why biologics are usually given by injection: the digestive system would destroy them. It is also why they cost more. A year of treatment with a monoclonal antibody can run into tens of thousands of dollars.

What defines a biologic:

• Source. Produced by living cells, not chemical synthesis.
• Size and complexity. Large, folded proteins with post-translational modifications.
• Route. Usually injected or infused.
• Regulation. Approved through a dedicated biologics pathway, not the standard small-molecule route.

Biosimilars, which are highly similar versions of approved biologics, have entered the market in recent years and brought prices down for some products.