Onset of action is the duration of time it takes for a drug's effects to come to prominence upon administration.
With oral administration, it typically ranges anywhere from 20 minutes to over an hour, depending on the drug in question.
A drug's pharmacological effects can only occur once it has been fully solubilized and has entered the blood stream.
Gastric emptying time can vary from 0 to 3 hours,[2] and therefore plays a major role in onset of action for orally administered drugs.
For intravenous administration, the pathway is much shorter because the drug is administered (usually already in solution) directly to the bloodstream.