Definition
Onset of action is the time at which the effect of a drug becomes observable or measurable. There is a delay between the administration of the drug (application) and the onset of action. We refer to this period of time as the latency period. It is in the range of minutes, hours, days, or even weeks if administered regularly. Some sources equate the latency period with the onset of action. However, this is not correct in our view. Duration of action is the period of time between the onset of action and the end of the effect.
Precondition for the effect
In order for a pharmacological effect to be triggered, the active substance must – as a rule – reach a drug target in the body. It usually reaches this via the bloodstream. The latency period is therefore shorter if the substance appears in the blood more quickly. Consequently, the dosage form and route of administration play a key role in the onset of action.
Dependence on dosage form and route of administration.
Intravenous administration is among the fastest routes of administration. The active ingredient is injected into the venous blood and spreads to its site of action within minutes. Inhaled administration is also generally characterized by a short time between application and effect. With smoking, for example, the psychoactive effects are triggered almost immediately. With tablets or capsules, it usually takes between half an hour and one and a half hours before an effect is felt. This is because the dosage forms must first dissolve in the stomach and intestines, and the active ingredient must be absorbed in the intestines. However, a further delay is possible with oral administration. The classic antidepressants only take effect after two to four weeks. This also applies to the lipid-lowering statins. If the release step is omitted in an oral dosage form, the onset of action may be faster. This can be observed with effervescent tablets, drops, dispersible tablets, or meltable tablets. With suppositories, the onset of action is usually delayed compared to tablets or capsules. This is not true, however, for a local effect, for example, against hemorrhoids or for constipation. When administered sublingually (under the tongue), the active ingredient is absorbed quickly and the effect occurs after a few minutes, for example, in the case of nitroglycerin capsules within two to three minutes.
Influencing factors
Latency depends on several factors (selection):
- Active ingredient: chemical structure, physicochemical properties.
- Pharmaceutical form, galenics
- Application type
- Application location
- Intake with or without food
- Disintegration time
- Absorption, bioavailability
- Metabolism
- Distribution
- Drug interactions
- Steady-State
- Drug target, mechanism of action
- Individual factors of the patient
The quickest possible onset of action is desirable in medical emergencies (e.g., emergency medications) and acute conditions such as headaches, among others.