Tamoxifen is a pro-drug which means that the active drug is produced in the body by the action of an enzyme - in this case a liver enzyme called cytochrome P450 2D2 (CYP2D2).  This liver enzyme is blocked by some drugs including a number of anti-depressants (including Prozac) which are commonly prescribed for the side effects of Tamoxifen treatment (eg hot flushes).  The anti-depressants work to reduce the side effects by preventing Tamoxifen from working. See the list of Tamoxifen drug Interactions.

A paper published last year looked at how many women prescribed Tamoxifen in Tayside actually took it.  And, what effect not taking it had on their survival.  Only 49% continued treatment for 5 years.  Tanking less than 80% of the prescribed dose was linked to a significantly reduced survival.  This translates to missing 1 tablet every 5 days.  Those taking aromatase inhibitors should also take note. Tamoxifen has a relatively long half-life so missing the occasional tablet is probably OK. However, the aromatase inhibitors have a much shorter half-life. See the original paper.

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