I just finished A2 having dropped Biology after AS when I realised sciences (continued with humanities) weren't for me but I have a friend who just finished AS doing those exact options. She got around 11 A*s at GCSE so our sixth form were confident with her taking it on. One downside is that she had next to no free periods, personally I found these essential to get work done by deadlines and exam practice shoved in nearer the exams. However, my exams were mostly essay based and so they also had coursework that took up a lot of time.
Honestly, I think it depends on just how good you are at the subjects. They link well together but, as others have said, you're better off getting AAAA first year than AABBB because you had to spread yourself over so many subjects. The course content can be vast, I'd recommend looking over what they're going to be exactly and how you will be tested. The step up is horrendous and definitely pokes holes in your knowledge of the subject (went from getting practically full marks in GCSE Bio to a B in AS and I put a hell of a lot of effort into it). Really you know yourself best. Are you motivated enough to work hard at all five? Do you have lots of things to do outside school that will prevent you from spending all your time on your subjects, can you cope with a high workload and is it really necessary for you to do all those subjects - even for medicine? (Seeing as the fourth A2 is discounted unless it is too low for you to get in, then suddenly you've lost your place). It's not really something anyone can judge for you.