Phenobarbital is a long-acting, polar drug that is slowly absorbed and slowly redistributed, contributing to its longer duration of action. However, phenobarbital is only partially converted and can be found unchanged in the urine. Most are rapidly metabolized to inactive compounds before they are excreted in the urine. High lipid-solubility in some (amobarbital, pentobarbital, and thiopental) allow them to be well absorbed and rapidly redistributed. The onset of action for oral administration ranges from 20 to 60 minutes, and intravenous administration can range from almost immediate to 5 minutes. Barbiturates have various onsets of action, durations, half-lives, and toxic levels, depending on the lipid solubility and rate of metabolic inactivation.
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