Need advice on blood donations will it be too much hassle?

So I've booked to give blood at the end of the month. I don't like needles at all but will they take blood if I'm prone to fainting or throwing up. I really work myself up about them to the point I pass out. I want to help out as I have a blood type which is needed in my area but don't know if I will be too much hassle?
Need advice on blood donations will it be too much hassle?
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