The author describes how her doctor-diagnosed "anxiety" was actually just triggered by excess caffeine from a specific brand.
This kind of misdiagnosis is not at all uncommon. We place doctors on a pedestal, and the overwhelming majority of them at least try to do an excellent job. But they are not magicians. They look for the simplest explanation without digging deep enough often. And, as Dear Abby once said, 50% of all doctors graduated in the bottom 50% of their class.
The same kind of misdiagnosis has happened to me multiple times. I had a serious case where I would become faint and even literally pass out at times. I was diagnosed as being hypoglycemic and having multiple food allergies, none of which was true. It turned out (I discovered on my own) that the symptoms were my body's reaction to indomethicin, which I was taking by prescription for inflammation from ballet dancing. Another time, I went to a doctor with a very sore top of my foot. I was diagnosed with "degenerative joint disease" and told that my joints would continue to deteriorate and hurt until I could not get around anymore. Actually, the problem was that I had gone on a long hike wearing Birkenstock sandals. They have no strap in the back and were a bit loose, so I was unconsciously flexing my big toe on every step to keep from kicking the sandals off my feet. That caused a brief overuse injury that rapidly went away.
Never believe anything a doctor tells you until you can corroborate it yourself.