This is what the official guideline for assisted dying in the Netherlands says about dehydration.
In assisted dying, it is important to first determine whether the patient is able to take the euthanasia drink. If a patient is dehydrated, the use of this method is not recommended.
Dehydration increases the risk of nausea, difficulty swallowing, and vomiting. When a patient is dehydrated, swallowing and absorbing the euthanasia drink can be harder, which may lead to incomplete or delayed absorption. This can make the process less effective and potentially more uncomfortable for the patient.
Dehydration also slows down digestion, which can delay the euthanasia drink's effect.
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Dehydration can begin within just a few hours, depending on factors like activity level, temperature, health, and age. Here's an outline of how quickly dehydration can set in:
1. Mild dehydration (within 4-8 hours): During intense activity or in warm temperatures, dehydration symptoms can appear in just a few hours if you don't drink. Symptoms may include thirst, dry mouth, slight fatigue, and less frequent or darker urine.
2. Moderate dehydration (after 12-24 hours): Going a full day without fluids can lead to more noticeable symptoms, such as dizziness, headache, difficulty concentrating, and dry skin. This can occur even without heavy activity, especially in warm or dry environments.
3. Severe dehydration (24-48 hours): After a day or more without water, symptoms become more serious, including confusion, weakness, rapid heart rate, and low blood pressure. The body can no longer maintain vital functions properly, which can pose significant health risks.
4. Life-threatening dehydration (after 48 hours or more): After two days with no fluids, the body enters a critical state. This can lead to organ failure, unconsciousness, and eventually death if rehydration doesn't occur.
In summary: Dehydration can start within just a few hours, especially during physical activity or in hot conditions, and can become severe within 24-48 hours.
Source
The Royal Dutch Pharmacists Association (Koninklijke Nederlandse Maatschappij ter bevordering der Pharmacie, KNMP)
https://www.knmp.nl/richtlijnen/uitvoering-euthanasie-en-hulp-bij-zelfdoding