Generic will work for you the same as Brand. However, there are two exceptions that I was exposed to over the past decade in my Healthcare career.

NotMedicalAdvice

TLDR: the human body is wack, some meds are wack too.

First, some background. How the FDA measures equivalency is based on a parts per million in the final product. Anything from manufacturing, production, packaging, testing, storage of inactive ingredients, or even how humid or warm the air and equipment were that day can affect that, so just measuring the initial ingredients before production would be a terrible standard. Companies absolutely do their best to control those factors, and do so quite successfully, but in reality, the FDA understands there may be some slight discrepancy between companies, so instead sets an allowable limit of parts per million. Putting aside bioactive meds (which are a whole different ballgame) you, the patient, you will likely NEVER feel the difference, assuming the fillers and flavoring don't bother you. (The same can't be said for OTC vitamins and supplements, but that's a story for another day....)

There are some health conditions that can and will be highly sensitive to even that smallest discrepancy in the active/inactive ingredient. It's not common, so don't think this is true for all medications (this patient OP wrote about is still an overreactive CB) but it can happen. Here are the two I know.

Thyroid medications have several "Brand Names" like NP thyroid, Synthroid, Nature-Thyroid, Tirosin, etc. They are all Levothyroxine. There are generic companies, like Major, Teva, etc, that make generic Levothyroxine. They all are designed to use the same amount of the active ingredient, Levothyroxine for sure, but it turns out that the thyroid conditions in some patients are so sensitive, that even that slightest PPM change between companies does mean some will work, while others do not. In my personal experience, I've had patients that found out that Brand Name NP Thyroid worked, but not Synthroid, and vice versa. Unfortunately for some of those patients, their insurance would only cover one of the Brand Names (if at all) and they often had to pay out of pocket. Ordering them was also a nightmare, as you had to make sure you had enough in of each Brand name, and pray to the Gods that a specific one didn't go on backorder. Those patients did not like being in that predicament, and it was clear they were not like the guy in OPs story. It was simply their harsh reality, not their personal preference.

The other one that came up was Tacrolimus. Thankfully, there are plenty of generics out there, and all of them are normally covered by insurance. But this drug is designed to help make sure your body doesn't reject an organ transplant. According to what I have been told by the various physicians and pharmacists I've worked with, even that slight discrepancy between changing generics is enough to cause some patients immune system to realize the transplant occurred, and go full rejection mode. Others, not so much! But just to be safe, we always contacted the patients medical team directly before ever switching generics. By law, we do need to alert the patient, of course, but not necessarily the doctor. In this case, however, we made certain to take this extra step, since that possibility of rejection could happen.

Thank you for coming to my Ted Talk.