Yes but no. Generics are not 1-1 substitutes because the FDA allows some (a lot) of wiggle room when it comes to bioequivalence.
A really good example where this fails miserably is Adderall for ADHD. The generics are absolutely miserable. And if you actually find a generic you tolerate you can never switch pharmacies because they’ll be different.
I’m no pharmacological expert but when I asked my pharmacist they explained it to me they said it was because Adderall is a mix of two drugs and it’s expensive to refine them into a balanced ratio but the FDA doesn’t care so generics don’t bother.
I believe it's therapeutic equivalence that the FDA is lax on, not bioequivalence. Bioequivalence is just "does it deliver the same amount of the active ingredient," theraputic equivalence is "does it deliver the active ingredient in the same manner." The active ingredient is the same, the inactive ones are where the difference lies. Usually, they're similar enough that it doesn't really matter, but some drugs really do benefit from buying the name brand.
A good example is with methylphenidate generics - you get the same amount of methylphenidate, but the name-brand extended release is far more effective than the generic extended release because it spreads the same amount of active ingredient over a longer period (the delivery mechanism created by the inactive ingredients in some generics is subpar).
Occasionally, you'll get non-bioequivalent generics that slip through FDA approval, which happened to some methylphenidate generics [0][1]. I remember getting switched to the name brand when this came out, as the supplier for all the pharmacies near me used a non-bioequivalent manufacturer. (Authorized generics don't have this problem since it's literally the name brand with generic labels slapped onto it, but the one the nearby pharmacies could get through their suppliers was not the authorized generic.)
ADHD medication is an excellent example illustrating the potential "wiggle room" differences between generics and brand drugs. The composition is similar enough to meet FDA generic drug standards, but the rate at which the generic drug mechanisms distribute the drug differs and potentially decreases treatment efficacy for many patients.
Though, I do wonder if Vyvanse becoming a generic brand will help solve this dilemma, because the release mechanism is tied closer to the individuals metabolism via hydrolysis of an amino acid rather than relying on release rates set by minor manufacturing differences.
The rules aren't strict enough IMO but the observed variation is very low in practice.
Australia allows very similar amounts of wriggle room in theory [1].
Most people I know in Australia with ADHD are treated with generics (very common among my friend group for some reason) and none that I am aware of have noticed any substantial differences between generics and brand name equivalents but I couldn't quickly find any studies that have investigated this.
I wonder if that is due to variation in balance of the included amphetamines (as most of these are multiple active ingredients) though as I doubt that the inactive ingredients would play a significant part.
A really good example where this fails miserably is Adderall for ADHD. The generics are absolutely miserable. And if you actually find a generic you tolerate you can never switch pharmacies because they’ll be different.
I’m no pharmacological expert but when I asked my pharmacist they explained it to me they said it was because Adderall is a mix of two drugs and it’s expensive to refine them into a balanced ratio but the FDA doesn’t care so generics don’t bother.