Venom with no cure? Bullshit. Replace all their blood as fast as possible.

Apart from what other people have said (low energy, you might get rejected) there's a tiny chance of other things going wrong. I used to donate whole blood every three months, now I donate plasma twice a week. - risk of vasovagal response/fainting. Some people just faint when they get poked, some people faint from the blood loss. It'll make you feel sick, but as long as you don't hit anything on the way down there shouldn't be any long term effects - pain/tenderness. Usually the actual needle spot doesn't hurt at all the next day. Whole blood hurts less than plasma. One time I think the phlebotomist hit a nerve or something and it hurt for a week after, but that's rare. Usually the finger prick before you donate is the most painful part, and sometimes that leaves a bruise for a long time (if you donate whole blood with red cross, they've started using a prick-less method to test iron levels) - low iron/protein. Blood needs a lot of iron, so if you donate whole blood regularly, you might end up with low iron. Taking supplements or eating iron rich food help. They also test your iron levels beforehand, so it should stay at a healthy level, but I think they test active iron instead of stored iron, so there's a chance you could still be low?? Or something like that idk. Donating plasma also takes a lot of protein so you'd need to eat more. - puncturing a vein. There's a small chance the phlebotomist will puncture a vein. Usually you'll get a tiny bruise out of it, but people have gotten way worse. This is especially bad for plasma, since they return some blood back to you. The machines are designed to stop if they sense something wrong, to make sure the blood goes into your veins and not into your arm spacer, but something can always go wrong - reactions. You can have an allergic reaction to anything, but you can also have a reaction to the anticoagulants if you donate plasma. In rare cases this can cause seizures. There are always a certain number of phlebotomists on the floor watching everyone, and no one is allowed to nap (they want to make sure you didn't fall unconscious because of a reaction). Usually you'll just get a tingly feeling or a bad taste in your mouth if you're having a reaction though. All of these are pretty rare though, and they're worth it to save someone's life. When you donate blood they give you a pamphlet about all the bad things that could happen, so it's not like they're trying to hide it.