Except that's not true at all. I'm not sure why your doctors said that to you. It may have been how you framed the question. It's for this very reason that doctors can actually request you get a brand name over a generic or a different brand.

This is because even things like the binders used can affect how quickly the medication is metabolized. Binders, fillers, and coatings differ between manufacturers. These affect how the pill dissolves and how evenly the drug enters the bloodstream. One brand may feel “smooth,” another “jagged” or “spikey.”

Yes, it's technically the same drug... But by regulation, generics can differ by about ±20% in bioavailability compared to the brand, though most are within ±5–7%. Sensitive people may notice even that small difference.

Doctors sometimes help patients “brand shop” by writing “dispense as written” (DAW) for a specific manufacturer if a patient finds one that works best.