This was taken from the National Blood Service website.
"You may not be able to give blood if:
1 You've had a serious illness or major surgery in the past or are currently on medication. Please discuss this with the clinical staff. The reason you're taking medicines may prevent you from donating.
2 You've had complicated dental work (although simple fillings are OK on the same day, as are simple extractions after 24 hours).
3 You've been in contact with an infectious disease or have been given certain immunisations in the last four weeks.
4 You're presently on a hospital waiting list or undergoing medical tests.
5 You do not weigh over 50kg (7st 12). If you are unsure please feel free to call our 24 hour donor helpline on 0845 7 711 711.
Pregnancy
If you are pregnant or you are a woman who has had a baby in the last 9 months."