Overview of Your Medication's Half-Life

The Rate That a Medication Is Eliminated From Your Bloodstream

Woman in bathrobe taking medication
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A medication's biological or terminal half-life is how long it takes for half of the dose to be eliminated from the bloodstream. In medical terms, the half-life of a drug is the time it takes for the plasma concentration of a drug to reach half of its original concentration.

What does half-life mean for dosage and medication use? Your diagnosis may be just one component of how your treatment is handled.

Your body has a say as well. How your medication interacts with your body will determine the frequency and strength of your meds.

Half-Life of Medication

Interestingly, no matter what the half-life of a medication is, it takes about four times that half-life for the concentration of a medication in the system to reach a "steady state." 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 that state at the beginning of the third day (after 48 hours). It may take anywhere from a week to two weeks to come off of certain medications due to its half-life.

How Half-Life Affects Medication Dosage and Administration

The goal of any medication is to keep it at a steady state so that it may remain balanced in your bloodstream.

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 work quicker but make it harder to come off of them and possibly could cause a dependency if taken over a long period of time.

Both the strength and duration of your medication will be considered before you are removed from a medication.

The half-life of the medication will be taken into account to determine how to lower your dosage and the frequency of when you take the medication.

Steady Withdrawal to Avoid Complications

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 an MAOI antidepressant.

View Article Sources
  • 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.