The half-life of a drug is an estimate of the time it takes for the concentration or amount in the body of that drug to be reduced by exactly one half (50%).
After four to five half-lives, 97% of a drug has cleared from the body, and the drug is no longer considered to be having an effect, although, for most drugs, actual noticeable effects would have worn off well before then. The symbol for half-life is T½.
For example, if 100mg of a drug with a half-life of 60 minutes is taken, the following is estimated:
- 60 minutes after administration, 50mg remains
- 120 minutes after administration, 25mg remains
- 180 minutes after administration, 12.5mg remains
- 240 minutes after administration, 6.25mg remains
- 300 minutes after administration, 3.125mg remains.
In theory, we can see that after 300 minutes, almost 97% of this drug is expected to have been eliminated. However, this does not mean that it won’t be detectable by a drug test, as this depends on how specific and sensitive the drug test is.
If you already know the half-life of your particular drug, you can use our half-life calculator here to get an estimate of the time it will take for that drug to clear from your body. Note that this is only an estimate, as we are unable to take into account patient-specific factors that apply to you.
What is the half-life for a particular drug?
Prescribing drug information usually states the half-life for that particular drug. It is usually presented as a range, for example, 11.2 to 26 hours, because, in reality, the half-life of a drug varies from person to person, and even sometimes within the same person.
See our article Drug Half-life Explained which lists the half-lives of some common drugs.
What factors affect half-life?
Patient-specific factors that affect half-life include:
- Age
- Blood circulation
- Diet (eg, grapefruit juice affects the half-life of some drugs; green vegetables affect the half-life of warfarin)
- Excessive fluid (such as in people with heart failure or edema) or low fluid levels (dehydration)
- Gender (sometimes)
- History of previous drug use
- Kidney function (for drugs that are cleared via the kidneys)
- Liver function (for drugs that are metabolized through the liver)
- Obesity
- Pre-existing conditions (such as heart failure, gastrointestinal disorders, pregnancy)
- Presence of other drugs that compete for binding sites or interact in other ways
- Race/ethnicity (this can influence the metabolism of a drug)
- Smoking (can also influence the metabolism of a drug).
Drug-specific variables that may affect half-life include:
- Drug formulation (ie, modified or controlled release preparations extend half-life)
- How the drug behaves in the body (ie, zero-order, first-order or multi-compartmental pharmacokinetics)
- How the drug is administered (half-life may be different with IV administration, compared to intranasal or oral administration)
- How the drug is cleared from the body (eg, kidneys, liver, lungs)
- If the drug accumulates in fat or other types of tissue
- If the drug binds to proteins or not
- Presence of metabolites or other drugs that may interact
- Properties of the drug, including molecule size, charge, and pKa
- The volume of distribution of a drug.
For more information see Drug Half-life Explained.