Because they are quickly metabolized by the liver.
Ultra short-acting barbiturates, such as thiopental, have rapid onset and very short duration of action, typically used for anesthesia induction, as they are quickly metabolized and redistributed. In contrast, short-acting barbiturates, like pentobarbital, have a longer duration of action and are often used for sedation or as anticonvulsants. The key difference lies in their pharmacokinetics: ultra short-acting barbiturates are rapidly eliminated from the bloodstream, while short-acting ones have a more prolonged effect due to slower metabolism and clearance.
Benzodiazepines have largely replaced barbiturates due to their improved safety profile and lower risk of overdose. Benzodiazepines have a wider therapeutic window and produce less respiratory depression, making them safer for use, especially in patients with respiratory issues. Additionally, benzodiazepines have a quicker onset and shorter duration of action, making them more suitable for treating anxiety and insomnia. The potential for dependence and withdrawal is still present with both classes, but benzodiazepines are generally considered less dangerous for long-term use.
site of action is the location (site) where the drug do there action ........ while onset of action is the starting of effects produced due to drug............... both are clearly different..........
Prophylaxis or primary prevention mean action delaying onset of disease.
Chronic disorders such as diabetes, hypertension, and osteoarthritis are characterized by a slow onset and long duration. These conditions typically develop over time and can persist for years or even a lifetime without a cure.
With amphetamine, as with any drug, the onset is the time after which you begin to feel its effects. For amphetamine, the onset time when taken by mouth is about 20 to 60 minutes. The duration is how long you can feel the effects of the drug, in this case about 6 hours, give or take.
Acute
Onset of .5-1 min IV lasting 2-3 min. Onset of 2-3 min IM lasting 10-30 min
30-90 minutes.
30 to 60minutes
Chronic diseases are those that have a slow onset and a long duration.
The onset of action for SSRIs (selective serotonin reuptake inhibitors) typically ranges from 1 to 4 weeks before noticeable improvement in symptoms is observed.