If you pour the Philosopher's stone into the blood of a human, the Stone might attempt to take over him/her. But I think it's possible that this will only work if the Stone is prepared by Father. There hasn't been many homunculi made this way (only two), so there's not much to say.
But, a doctor with knowledge in using alchemy for healing with a Philosopher's Stone, can use its value in human lives as payment to the Equivalent Exchange, and in that way enhance his abilities, so that he can heal almost anything.
That's how it works.. I think. :S