I got this advice from a reputable organization specialised in terminal relief:
buy some sodium nitrite E250 at your local butcher or on the internet. The salt is very soluble in water. To prepare a lethal dose of the salt, at least 5gm is dissolved into 50 ml of water. There is no significant adverse taste and vomiting is unlikely. However, it is advised that an anti-emetic ( 4 x 10mg of metoclopramide - Maxolon) be taken as a single dose, 30 minutes before drinking the nitrite solution.