The four people who died using the ROTI Act all injected themselves with Nembutal (with the help of the Deliverance Machine, now on display in the British Science Museum). See: http://en.wikipedia.org/wiki/File:Euthanasia_machine_(Australia).JPG
While these people could also have simply drunk the liquid Nembutal, each preferred intravenous administration. When delivered in this way, loss of consciousness is almost immediate (seconds), with death following a short time later.
Drinking Nembutal is often the preferred option and means no other person need be involved in administration. For example, in the state of Oregon in the US, a doctor is only allowed to prescribe as opposed to administer a 10 gm oral dose of barbiturates to a patient. The patient must drink the drug themselves. In Switzerland, too, it is the client who must administer the drug him/her self. In Holland and Belgium, it is lawful to provide barbiturates as an injection to a dying patient. The drug used in each these places is Nembutal.