Onset of action is the duration of time it takes for a

drug A drug is any chemical substance that causes a change in an organism's physiology or psychology when consumed. Drugs are typically distinguished from food and substances that provide nutritional support. Consumption of drugs can be via insuffla ...

's effects to come to prominence upon administration. With

oral administration Oral administration is a route of administration where a substance is taken through the mouth. Per os abbreviated to P.O. is sometimes used as a direction for medication to be taken orally. Many medications are taken orally because they are in ...

, it typically ranges anywhere from 20 minutes to over an hour, depending on the drug in question. Other methods of ingestion such as

smoking Smoking is a practice in which a substance is burned and the resulting smoke is typically breathed in to be tasted and absorbed into the bloodstream. Most commonly, the substance used is the dried leaves of the tobacco plant, which have bee ...

injection Injection or injected may refer to: Science and technology * Injective function, a mathematical function mapping distinct arguments to distinct values * Injection (medicine), insertion of liquid into the body with a syringe * Injection, in broadca ...

can take as little as seconds to minutes to take effect. The determination of the onset of action, however, is not completely dependent upon

route of administration A route of administration in pharmacology and toxicology is the way by which a medication, drug, fluid, poison, or other substance is taken into the body. Routes of administration are generally classified by the location at which the substance i ...

. There are several other factors that determine the onset of action for a specific drug, including

drug formulation Pharmaceutical formulation, in pharmaceutics, is the process in which different chemical substances, including the active drug, are combined to produce a final medication, medicinal product. The word ''formulation'' is often used in a way that incl ...

, dosage, and the patient receiving the drug.

Effect of Administration Route on the Onset of Action

A drug's pharmacological effects can only occur once it has been fully solubilized and has entered the blood stream. For most drugs administered orally, the drug must be ingested, pass through the stomach, and into the small intestine, where the drug molecules enter the blood stream through the villi and

microvilli Microvilli (singular: microvillus) are microscopic cellular membrane protrusions that increase the surface area for diffusion and minimize any increase in volume, and are involved in a wide variety of functions, including absorption, secretion, ...

. A few drugs such as alcohol are absorbed by the lining of the stomach, and therefore tend to take effect much more quickly than the vast majority of oral medications which are absorbed in the small intestine. Gastric emptying time can vary from 0 to 3 hours, 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.

References

Pharmacokinetic metrics {{pharmacology-stub