It can't be emphasized enough for me just how unpleasant side effects like heart palpitations, tachycardia and related conditions really are. For me at least, those kinds of conditions trigger something close to a panic attack which feels like a very deep seat primitive protective measure in our brains.

Laying in your bed trying to be restful and feeling your heart slamming away at 140 beats per minute with little you can do about it goes behind miserable into outright alarming, even if you are "used" to it.