If you’re a layperson, this is the way you probably think of generics: They’re the exact same products in different packaging.

The FDA agency’s definition of bioequivalence is surprisingly broad: A generic’s maximum concentration of active ingredient in the blood must not fall more than 20% below or 25% above that of the brand name. This means a potential range of 45%, by that measure, among generics labeled as being the same.

There are other differences. The generic must contain the same active ingredient as the original. But the additional ingredients, known as excipients, can be different and are often of lower quality. Those differences can affect what’s called bioavailability — the amount of drug that could potentially be absorbed into the bloodstream.

These and other factors need to be taken into consideration, especially if your medicate is designed to save your eyesight or other vital organs.