@iserbrown and @Mira, I also had an awful experience with a different brand of Tamoxifen. Within days of changing (my usual brand unavailable) I was bursting into tears for no reason and had aches and pains like I had never experienced on my usual brand. Apparently different brands can have different ingredients. The 'active' ingredient or medicine is the same, of course, but the things they mix in can be very different. There was another lady on here who developed a rash with a different brand too.
So I guess if you are having a lot of side effects, changing brands is worth a try. If you have one that seems to suit (I had almost no side effects most of the two years that I was on Tamoxifen) then stick to it!