Please check out this site.
Tamoxifen and Fertility
Tamoxifen increases ovulation and doesn't seem to effect unfertilized eggs, but WILL effect fertilized eggs. I assume that your RE prescribed it for you BEFORE you ovulate, but then you are going off it after? If this is the case, you should be okay.
Is this a last resort drug for you? Can you take something else? I am just concerned due to all the warnings...
Oh, here is another link I found:
How Does Fertility Treatment Work?
Seems that it is being EXPERIMENTALLY used in high doses for short periods of time to boost egg production- but looks like you'd want to do that for IVF, not IUI...