Thing about Psychology, like others have said, is that you either love it or you hate it - and the majority of people hate it. In theory, Psychology is fascinating; unfortunately, at AS/A2 Level, the topics aren't relevant to anything and it is one hell of a lot of stuff to remember. It's not like some humanities subjects (like English, for example) where you can sit the exam without a great deal of subject knowledge and still do well based on essay technique etc. Psychology (at AS at least) is basically entirely content. I'd advise against it if you want to do medicine.