Me-too pharmaceutical products: History, definitions, examples, and relevance to drug shortages and essential medicines lists.

We define a me-too drug as a pharmacologically active compound that is structurally related to a first-in-class compound, regarded as belonging to the same therapeutic class as the original compound, and used for the same therapeutic purposes, but which may differ in some respects, such as specificity of pharmacological action, adverse reactions profile, or drug-drug interactions. We also offer definitions of related terms, including follow-on drug and first-in-class. The therapeutic advantages of me-too drugs may include improved target specificity, reduced risks of off-target adverse reactions and drug-drug interactions, increased chance of benefit in some patients, and improved drug delivery and pharmacokinetics. Me-too drugs can also demonstrate incremental innovation. Their availability may help in coping with drug shortages. However, they may occasionally cause unexpected adverse reactions that are not class effects. Tricyclic antidepressants, β-blockers, and statins illustrate the diversity of me-too drugs. Earlier compounds may be as effective as later ones, or more so. Tricyclic antidepressants have similar chemical structures, and compounds introduced after the first-in-class compound (imipramine) mostly offered little in the way of innovative features, but continue to be prescribed. In contrast, me-too β-blockers introduced after the first-in-class compound, pronethalol, have diverse structures and display several innovative features. Stereoisomers and biosimilars/biobetters provide special examples of me-too drugs. Although many me-too drugs offer no significant advantages over their predecessors, over 60% of the drugs listed on the World Health Organization's essential list are me-toos. Different countries may choose different me-too drugs when constructing essential medicines lists, partly explaining transnational differences between them.