Ms Purdy welcomed the intervention by the DPP.
She said it was important to underline that people considering suicide had a duty first to carefully consider all possible options.
"People will know what they must make sure of before they assist, and hopefully that will give people confidence not to make such a decision until the last possible minute."
And Sarah Wootton, chief executive of the Dignity in Dying campaign group, agreed, saying the guidance represented a "significant breakthrough".
BBC NEWS | Health | Assisted suicide law 'clarified'