As a result of suffering three of the above conditions for several months, possibly years, I have thrown every ounce of energy into concentrating on what needs to be done. I have developed massive concentration which leads to Worst Side Effect #1: I am relentless.
As in, utterly relentless. I write, so I evaluate every side of the situation and every character's response to it, although I may only write in a few. That leads to great characterization, but it is an enormous waste of time and energy, fighting the fog to put myself in each character's shoes.
What is worse is being relentless 'in real life'. I have lost all capacity to judge when a conversation should be over, and beat the subject to death until the listeners run screaming from the room. When I start 'figuring' and 'estimating', it's time to take me down bodily.
I have a very odd sense of humor, and when I unleash it, relentlessly, I have people pissing their pants if not staring at me like an alien comedienne who doesn't understand the culture yet. And God forbid I try to make someone feel better! They start to wonder why, as if they were rich people and I am looking to get put in their will!
Brain fog's #2 Worst Side Effect: I can't keep track of my life. If I don't know how much money I've spent, I spend it until it's gone. I can't remember what promotions I have done, despite drawing up lists on my computer; those lists are undoubtedly in several different folders. I virtually live off my computer, so I document everything, and frequently run searches (if I can remember the keyword!).
I must have a detailed Google driving map to get anywhere, and I often get lost even then, so I usually only drive routes I know very well. I will walk by signs saying Do Not Enter, and walk by people in distress because I am hell-bent-relentless on getting to my destination.
I am amazed that this horrific infection I had on my finger healed so well, but then again, I type all day long and probably over-treated it relentlessly with three different strategies...! I am lucky my husband puts all my appointments on his iPhone; hell, I'm lucky he remembers to feed me! If I did not also have my magnificent husband, I'd be deader than a doornail, having forgotten to eat, sleep, taken a shower, pay the electric bill, and a host of other must-do's around the house that leads to safe living.
So please, if you know someone who suffers from occasional (or even 'relentless'!) brain fog, cut them a lot of slack. They are not stupid; they are suffering! Get with, or be, their caretaker if they want one and you have the time. Understand that it may be temporary -- I was ever so thrilled to get rid of a Candida infection I had for God knows how long! -- or it may be part of a worsening illness. It's not that we don't care; it's that we are not able to concentrate, be aware, or notice!