You have no idea the relief Ive felt to find out the twitching was hormone (and anxiety) related and not something more sinister. I figured it out this way - when I went on tamoxifen a year and a half ago, I had a rather extreme response, with awful complete oestrogen depeletion, and doc said it shut down my oesterogen almost totally and immediately (which isnt meant to occur, it works by a different mechanism). My muscle twitching at that point was to the point of random large scale 'jerking' like spasms which could obviously be seen by anybody clearly lol, after stopping tamoxifen it went back down to the random muscle twitching which has remained......and I began the journey of Peri kick started by tamoxifen. So, I can confirm to anyone worried about this twitching, its hormone related (and probably tiredness too lol) . I didnt realise but muscles do make some oestrogen.