Some drugs that are not well absorbed through the gut when taken orally (potassium for example), can cause death when administered intravenously.
Drugs are occasionally also administered rectally using suppositories, or by direct infusion (enema). This is usually done if there is difficulty swallowing or if vomiting is a problem. Some lethal drugs can be quickly absorbed in this way. Rectal administration can provide a means of proceeding if there are intractable difficulties associated with oral administration.
The act of taking a lethal drug does not generally result in an immediate death. Rather, the time that elapses from consuming the drugs until death, depends on a number of factors. This time in between administration and death can occasionally lead to failure.
Some drugs or substances, when taken orally, act very quickly. In some cases, speed of death can be an important factor. Such as when a spy takes a suicide pill to prevent interrogation or torture. For example, Hermann Goering took a cyanide pill in his cell the night before he was due to be executed. Although Goering was being watched very closely, his death was so quick that resuscitation was impossible.
A very rapid death - Goering-style - is rarely a consideration for a seriously ill person. Rather the readers of this book are more likely to think of a peaceful death as dying in one’s sleep.