There are none, unless you're not healthy enough to be donating in the first place. Some people who are donating for the first time get really nervous and faint, but that's not really harmful. The needle only hurts a little little bit, and only for a second. And what are the good effects of donating? 1. You can help save 3 lives with every donation. 2. You get free juice and cookies after you donate. 3. If you need blood for surgery of your own, and you can prove that you've donated more than that, you can get the blood you need for free. (Hospitals usually charge about $120 per pint for blood you use in surgery). I'd say the positives outweigh the negatives.
If you're tall & healthy, of average or more weight, a slightly sore arm for a day or two, that's all. (Some people feel a little dizzy & light-headed afterward but it usually only lasts an hour or so. Drink a large glass of juice or water in the morning, beforehand, so you're hydrated. It helps.)
well losing blood isnt a good thing so heres the list, insufficient oxygen flow, death, and even worse, brain cancer aaaaaaaaaaaand thats pretty much it.......trust me my uncle is like a doctor or something for plants.