I've only donated once and my experience wasn't great. I would do it again, but not at the same campus I got it done at.
If they give out free food I'm more likely to do it as I feel woosy afterwards and being able to eat something really helps.
They are really careful about that stuff and don't want you passing out.
Every time I donate, I am
always given food and water. Always. Before and/or after, depending on how hungry I am. I usually donate to the Red Cross or some other organization that appeared on my college campus.
I said I was a little dizzy once (it really wasn't that bad) and the nurses were all over me. They told me to lie on the ground, so I was just lying there with my legs elevated and everything despite insisting I was fine lol.
So I don't think they'd leave you hanging; they do ask if you are a little dizzy or woozy afterwards.
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Anyway, I've donated over the years and it's really nothing. Just wait to be called, lie down, donate, then eat/drink.
I don't recommend donating blood on a really hot day because you can pass out.
I've also been rejected once for being low on iron, but it was because I was in jogging mode every other day.