Thank you all so much for replying! This is really interesting - I think there is definitely something in this and seemingly this area has been studied as it has been noted that we react and metabolise drugs differently. There are some very interesting scholarly articles out there about this.
Yes, Sparkle - it's the negative part that's different. We have no rhesus factor, which makes us unique in many ways. A fascinating throwaway fact I found out is that it is possible to clone rhesus positives but it would be IMPOSSIBLE to clone a rhesus negative person. There is a lot of science behind this, and again there are some good articles out there.
I'm exactly the same, Lizab and Maryjane - even post operatively, I require much smaller amounts of pain relief as it affects me quite dramatically. The hospital pharmacist was most surprised too when she wrote up my pain medications to take home. I needed only half of the normal dose of Tramadol, and even this was a bit too much!
Oh, and Lizab - red hair is a common trait of rhesus negatives too!