How does one know whether the spread of cancer was as a result of taking
Tamoxifen?  Do you have a reason to suspect that such a causal link may
exist?

Yes, my wife was given Tamoxifen, and her cancer later spread, but I
have no reason to blame Tamoxifen for the spread, I think it was more
likely due to the cancer.  You can read our stories at
www.tim-jackson.co.uk/cancer.html

Tim's point is right on.  "Post hoc ergo propter hoc"--loosely, after the
thing therefore because of the thing--is a common fallacy.  Tamoxifen is
given because it reduces the frequency of recurrence in groups of people.
Reduces doesn't meean absolutely prevents, so you would expect fair numbers
of people (men and women) who have taken tamoxifen for breast cancer would,
nevertheless, have recurrence.  Tallying up such examples would not
constitute proof, nor even much credible evidence, that the tamoxifen caused
the recurrence.