I donate (or try to) every 8 weeks. My iron is frequently low - not low for the general business of life but low for donation. They always suggest taking a children's chewable vitamin with iron each day for a week or so before donating. It does help but I frequently don't check the calendar far enough in advance.
I look at it as a charitable donation AND you get cookies and juice! My husband once won a $50 gas card for entering a drawing at the blood bank!