I used to donate regularly but then I went on some kind of skin medicine that can cause birth defects if a pregnant woman got my blood - no donating for THREE YEARS after I quit taking it.

I would donate "double reds" so it would take about two weeks before I didn't notice during intervals, and a couple more before I couldn't tell at all.

Bonus - the nurse offered to take my HCT before and after so I could see the drop... 43.8 to 38.7 last time I went in.