@GoddersUK: Don't forget an MD - a Doctor of Medicine, which is another type of doctorate that requires 6 years of education. You can get a PhD in Medicine but it's pretty rare.
But, to add to the confusion, a Doctorate of Medicine isn't required to be a medical doctor (in the UK, at least). Rather the requisite qualifiation for that is the MBBS (Bachelor of Medicine, Bachelor of Surgery) (which is also a prequisite to undertake an MD). However a US MD is equivalent to a UK MBBS. Moreover, at a few universities, an MD is apparently the medical equivalent of a DSc, rather than the more traditional research degree it is elsewhere.