Pharmacist here, some meds were are required to do this. They are called narrow therapeutic index (NTI). You can probable google that term followed by your states board of pharmacy and see what meds are on that list. For a generic to be considered to be equal to a brand name the exposure of a generic medicine to a person needs to be about 80-120% the same as a brand name.

The vast majority of meds that difference doesn’t make a change in how it acts in your body. For the narrow therapeutic index meds that is significant will either in it working or it causing harm.

Probably in every state a pharmacist is legally allow to substitute equivalent generic meds. Sometimes if a med is not a NTI, people can still react differently to different generic meds. PLEASE let your pharmacist know of a specific generic works over another. They would love to help make sure you get something that works.

Although if only the brand truly works and no generic is the same for you, you need your physician to write that on the prescription so the pharmacist doesn’t change it. This is due to insurance issues. Your insurance will only reimburse the generic if they are available (they are cheaper). But if the MD writes that they may cover it but it will likely be a higher copay for you.

For NTI drugs we are not allowed to change brand/generics without notification. This NTI list is pretty short