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What does "Onset of Action" Mean?

Mary McMahon
Mary McMahon
Mary McMahon
Mary McMahon

Onset of action is a medical term referring to the amount of time it takes for the effects of a drug to become noticeable after administration. Some medications have an extremely rapid onset, with people noticing activity within seconds. Others, like antidepressant drugs, can take days or even weeks to fully take effect. When pharmaceutical companies develop new medications, one of the things they study is the onset of action, to determine how quickly the drug takes effect.

The route used for delivery can make a big difference. For very rapid onset, intravenous medication is often the best choice because it enters the bloodstream directly and the patient does not need to metabolize it to make the drug available. Drugs applied to the mucus membranes can also act very quickly, as the medication quickly seeps into the capillaries and enters the bloodstream. Oral medications work more slowly, plus stomach acid can interfere with some drugs, making this route impossible. Other options can include suppositories, topical creams, and inhalation drugs.

Medication administered through an intravenous has a rapid onset of action.
Medication administered through an intravenous has a rapid onset of action.

Drug formulation can also be a factor. Many drug companies make extended release formats, allowing people to take a medication and have it slowly take effect over a period of hours. This slow onset can be useful for people who need to take drugs for maintenance of medical conditions. They may take fewer pills because they act longer, and are at a lower risk of an overdose. The delayed delivery can also keep levels of the drug in the body more stable, reducing issues like spikes or falls in concentrations of the medication.

Oral medication tends to have a slow onset of action because it must dissolve in the stomach to be released into the blood stream.
Oral medication tends to have a slow onset of action because it must dissolve in the stomach to be released into the blood stream.

Some drugs with a usually rapid onset of action include sedatives and pain management, where the goal is usually immediate relief for the patient. With drugs for mental health conditions, the drug may take effect slowly because the patient's brain chemistry needs to change for there to be observable effects. Treating conditions like depression and bipolar disorder with medication is not as simple as addressing a problem like inflammation or pain, where a drug can be delivered right to the site to take effect very quickly.

Antidepressant medication has an extremely slow onset of action as it may take days or weeks for patients to feel its full effect.
Antidepressant medication has an extremely slow onset of action as it may take days or weeks for patients to feel its full effect.

Onset of action is also a consideration when doctors think about how long a drug will linger in a patient's system. Drugs that take effect fast tend to clear the body very quickly, while slow-acting drugs take longer to fully metabolize. This is a concern when evaluating patients for potentially harmful drug interactions; a patient may not be on a medication anymore, but could still be at risk of a reaction, for instance.

Mary McMahon
Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

Mary McMahon
Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

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Discussion Comments

Inaventu

I took an herbal supplement that was supposed to help with anxiety, and the instructions on the box said it would take twenty minutes for the pills to take effect. Sure enough, I felt much calmer right around the twenty minute mark. I suppose doctors and pharmacists have figured out the onset of action for most prescription drugs by now.

I know when I took xanax, the onset of action was a few hours, but when it finally kicked in, it was very noticeable. When it was time to wean myself off that medication, my doctor wanted to make sure it was completely out of my system before I started taking a different anti-anxiety drug.

mrwormy

I have seen some commercials for heartburn or stomach acid relief pills that really play up the "onset of action" difference. People in gastric distress often seek immediate relief, which puts antacid tablets ahead of acid reduction medications. While it might be better in the long run to take an acid reducer, the pills have to be digested before they can get into the blood stream. Antacids may not provide long-term relief, but they start working right away.

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    • Medication administered through an intravenous has a rapid onset of action.
      By: Tyler Olson
      Medication administered through an intravenous has a rapid onset of action.
    • Oral medication tends to have a slow onset of action because it must dissolve in the stomach to be released into the blood stream.
      By: amawasri
      Oral medication tends to have a slow onset of action because it must dissolve in the stomach to be released into the blood stream.
    • Antidepressant medication has an extremely slow onset of action as it may take days or weeks for patients to feel its full effect.
      By: James Steidl
      Antidepressant medication has an extremely slow onset of action as it may take days or weeks for patients to feel its full effect.
    • Medication given to relieve pain tends to have a quick onset of action so patients feel its effect soon after taking it.
      By: BlueSkyImages
      Medication given to relieve pain tends to have a quick onset of action so patients feel its effect soon after taking it.