Drug Half-life Explained

Medically reviewed by Carmen Pope, BPharm. Last updated on May 23, 2022.

What is the half-life of a Drug?

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%). 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. Most drugs are considered to have a negligible effect after four-to-five half-lives. However, this does not mean that won’t be detectable, for example, during a drug test. Just that they will have no effect.

In reality, the actual half-life of a drug varies from person to person, because it depends on a number of different patient- and drug-specific factors. These affect how well a particular drug is distributed around a person’s body (called the volume of distribution), or how fast a person excretes that drug (called the drug clearance). For example, the IV drug gentamicin, which is cleared through the kidneys, has a half-life of 2-3 hours in a young person with no kidney disease, but its half-life is over 24 hours in somebody with severe kidney disease.

Generally, it is difficult to precisely say how long a drug or substance will take to be excreted from someone’s body. This is an important fact for athletes or people in occupations that require them to be substance-free to remember. Half-lives in the anti-doping world are of limited value because they do not reflect the presence of metabolites (break-down products from the parent drug), which are often what is measured in anti-doping tests. In addition, serum half-life does not necessarily reflect urine concentrations, which is the main way they take samples for drug testing.

Patient-specific variables that may affect half-life

• Age
• Blood circulation
• Diet (eg, grapefruit juice and several drugs, green vegetables, and warfarin)
• Excessive fluid (such as in people with heart failure or edema) or low fluid levels (dehydration)
• Gender
• 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 drugs that compete for binding sites or interact in other ways
• Race/ethnicity or genetics (this can influence the metabolism of a drug)
• Smoking
• Other variables, such as if the person is on hemodialysis.

Drug-specific variables that may affect half-life

• 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
• Other variables, such as if the drug is actively transported, is self-induced, or has saturation pharmacokinetics.

Short versus long half-lives

Drugs or substances that have a shorter half-life tend to act very quickly, but their effects wear off rapidly, meaning that they usually need to be taken several times a day to have the same effect. Drugs with a longer half-life may take longer to start working, but their effects persist for longer, and they may only need to be dosed once a day, once a week, once a month, or even less frequently.

When considering drugs with a high addiction or dependence potential, those with a short half-life are typically harder to withdraw from than those with a long half-life. For this reason, drug treatment programs will often switch a person from a short-acting drug to a long-acting equivalent from the same class, in order to improve the withdrawal process.

List of common medicines or substances and their half-lives

* Note that half-life varies depending on the source used. Half-life in this table refers to the elimination half-life.

See also

• Common Drug Side Effects
• Does grapefruit juice interact with my medications?
• Drug Expiration Dates - Are expired drugs still safe to take?
• Generic vs Brand Drugs: Your FAQs Answered
• How do I remember to take my medications?
• How do I stop my medication safely?
• How to Safely Dispose of Your Old Medications
• Imprint Code FAQs - For Oral Medications
• Injection Types and Sites
• Is it safe to split my pill in half?
• Medical Conversions - How many mL in a teaspoon?
• Top 150 Prescription Abbreviations & Medical Meanings
• Top 5 Ways to Avoid Drug Errors
• Top 9 Ways to Prevent a Deadly Drug Interaction
• What are pharmaceutical salt names?
• What are the risks vs. benefits of medications?
• What is the placebo effect?

Further information

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.

Medical Disclaimer

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