Overview of Your Medication's Half-Life

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When doctors write prescriptions for medications, they don't just jot down the name of a drug on a little blue pad and send their patients off to the pharmacy. Included on the prescription are details about how much of the medication to take at one time (the dose) and at what intervals. These instructions, which are very important for making certain the drug is both effective and safe, are based in part on the half-life of the medication being prescribed.

As a patient, it rarely is necessary to know the half-life of a drug your doctor wants you to take, but it can help to understand what this term means and how it might affect you during the time you're on the drug.

Half-Life Defined

A medication's biological half-life refers simply to how long it takes for half of the dose to be metabolized and eliminated from the bloodstream. Or, put another way, the half-life of a drug is the time it takes for it to be reduced by half.

For example, the half-life of ibuprofen (the active ingredient in brand name pain-and-fever relievers such as Advil and Motrin) is about two hours. This means if you take a typical dose of 400 milligrams (mg) of ibuprofen at, say noon, half of the dose, 200 mg, will have been eliminated from your bloodstream by 2 p.m. By 4 p.m., 100 mg will have been eliminated, and so forth.

It's important to note that the expected half-life of a drug will vary from person to person, depending on factors such as age, weight, genetics, and even specific health issues. For example, the half-life of acetaminophen (the active ingredient in Tylenol, another non-narcotic pain relief medication), can be significantly affected by a person's liver function since acetaminophen is primarily processed through the liver.

Achieving a Steady State

The goal of any medication that will need to be taken on an ongoing basis, such as an antidepressant, is to get it to a "steady state"—in other words, to the point at which the amount that goes into the body is equal to the amount that's eliminated.

Interestingly, no matter what the half-life of a medication is, it takes about four times that amount of time for the concentration of the drug to reach a steady state in the body. This means that if you begin taking a medication with a half-life of 24 hours, after four days, or on the fifth day, the rate of intake of the drug will approximately equal the rate of elimination. If the half-life is 12 hours, you'll reach a steady state at the beginning of the third day (after 48 hours).

Why Half-Life Matters

Drugs with a longer half-life take longer to work, but on the positive side, they take less time to leave your bloodstream. On the flip side, those with a short half-life become effective more quickly but are harder to come off of. In fact, drugs with very short half-lives can lead to dependency if taken over a long period of time.

A drug's half-life is an important factor when it's time to stop taking it. Both the strength and duration of the medication will be considered, as will its half-life. This is important because you risk unpleasant withdrawal symptoms if you quit cold turkey.

Withdrawal symptoms are caused by quickly getting off of some types of medication. When you are being weaned from this type of medication, the drug's half-life will be considered so that those with a longer half-life will take longer to come off of. Medication side effects occur usually when the blood level of the drug is not in its steady state. That's why it's important to follow the dosage and duration recommendations to the letter. Otherwise, the body will react and the effect of the drug will be either toxic, as in more than intended, or not therapeutic, as in ineffective for treatment.

One impact of half-life is found in the SSRI antidepressants. People taking SSRIs with short half-lives are much more likely to experience SSRI discontinuation syndrome. People taking an SSRI with a long half-life such as Prozac need to wait far longer between stopping Prozac and starting a new antidepressant, such as an MAOI.

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Article Sources

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  • Roden DM. Principles of Clinical Pharmacology. In: Kasper D, Fauci A, Hauser S, Longo D, Jameson J, Loscalzo J. eds. Harrison's Principles of Internal Medicine, 19eNew York, NY: McGraw-Hill; 2015.