I want to get to 50 donations - have done 45, and was almost ready to give again when my medication got tweaked again. I can give again when it's been stable for four or six months (need to check - I was about to hit 4 months.)
There's a list of common medicines on the blood donation website, and whether you can donate, but it's not exhaustive, so if yours isn'the listed or you're still not sure, there's also a number you can call, and they were pretty good when I rang. They couldn't tell me straight away, but rang back quite quickly.
The rules around who can and can't donate are based on research, which is why they change frequently. They screen for more diseases than was possible when I first donated in the early '90s. The rules on when you could donate after travelling to malarial countries seemed to change every donor session at one point (when it was relevant to me.) The contaminated blood scandal also produced a lot of changes. There are still things they can't screen for, and the rules tend to err on the side of caution. It means we've got a pretty safe donation system, but there are still lots of things we just don't know yet. It's why research scientist's have jobs. Research is also why they now encourage people to drink water and do some sort of exercise, be it clenching your number or whatever.
The website is pretty informative about lots of aspects of donation.
If you can't donate yourself, you can help others who are able to, by giving lifts or minding children or something.
Whoever asked about a whitey - it's fainting, though not always completely blacking out. I have only fainted a couple of times in my life, and my vision went sort of very bright and white before the blackness started taking over my whole vision (happens quite quickly, though.) I also felt sort if tingly and sounds went distant, and those looking at me said I went very pale - white. My face is almost never pale. You can also throw a whitey by overdoing certain recreational drugs.