Any substance taken orally can be vomited up, and concern about this can cause considerable anxiety. A person intending to die must take the full (lethal) amount, so it is important to ensure that vomiting does not occur. Some people are prone to vomiting, and some diseases can cause vomiting. In a minority of cases vomiting, or even the fear of vomiting, can be such a problem that it is not possible to use oral drugs.
To make matters worse, end of life drugs are often bitter. Their strong taste, especially in liquid form, can induce vomiting. This will inevitably slow gastric emptying and effectiveness of the drugs.
To minimize the risk of vomiting and to speed gastric emptying, an anti-vomiting (‘anti-emetic’) drug can be taken as a single dose. Alternatively the drug can be taken for a set period of time before the consumption of the lethal drug. Indeed, organisations like Dignitas in Zurich, who provide oral Nembutal for those wanting to die, insist that an anti-emetic be taken prior to the lethal barbiturate. There is a number of anti-emetic drugs that can be used for this purpose, although they are often controlled/ prescription drugs. Be aware that it may be necessary to fabricate a reason when asking a doctor to prescribe them for you.
An additional point is that all anti-emetic drugs have side effects. Some can be serious. If you have never taken these anti-emetics before it would be wise to take a test dose of the planned drug prior well ahead of your dying day. This way any allergies and hypersensitivities to a particular anti-emetic occur can be taken into consideration in your overall planning.