I also had/have phantosmia and i read other msgs in this forum talking about this (ammonia smell/less smell, etc).
I just crashed 3 weeks ago from dutasteride. Seems this trouble with smell Its subsiding. I was also taking xanax first days (i wake up gaspering for Air at night cause of anxiety and also had palpitations and heart pain induced by my anxiety )and found that xanax can alterate smell.
I think that It can be for my case:
-anxiety (i’ve read that can cause phantosmia, with chemical/ammonia smells and my Psychiatrist confirmed that)
-sinisitis (also bad smells)
-kidney problems provoques ammonia breath(i also have some times high urinate frequency). But i had two Blood tests the first week when i crashed and carnitine was in range (but i Will haber hormonal/basic Blood tests again)
-xanax (It was stronger after using xanax).
-hormonal changes
I had various visits with medics and no one was worried at all and thinked about brain damage (even pshquiatrist)
I think that Your taking the most pesimist view. Thats not good for anxiety my friend! If Its long term effect in brain why subsides?
I wish you the best my friend.