Frans wrote
donating blood costs nothing more than your time and the slight discomfort of a 2mm needle. Its a worthwhile thing to do, the only louzy part is that they nag you to donate every 2 months. And most blood banks keep stupid hours.
They usually set up donor sites at shopping malls on weekends. There's a fairly regular one at Cresta.
I'm up around 40, but I can't give any more because I have to take anti-coagulants for the rest of my life ☹
But seconded (or thirded), it costs you nothing but time, you usually get a cup of tea and a couple of biscuits, the physical discomfort is minimal, the donation process itself is safe and they won't take your blood if they have reasons to believe it will cause problems for you. They will check to see if you're anaemic. If they see you are not well they will decline your offer and so on.