# Drug Half-life Explained

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

## Drug Half Life / Clearance Calculator

Work out how long it takes for a drug to leave your body.

Half Life:

### Half life Clearance

Drug Level Remaining Time
100.00%
50.00%
25.00%
12.50%
6.25%
3.13%

This is an estimate on the time it will take for a drug to be removed from the body.

The actual half-life of the same drug may vary significantly from person to person, because it depends on a number of different patient-specific and drug-specific variables.

This tool should NOT be considered as a substitute for any professional medical service, NOR as a substitute for clinical judgement. It should not be relied upon to predict the time period required to ensure a negative drug test result, because laboratory tests mostly test for drug metabolites.

## 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

Generic Name Brand Name Examples Half-life (T1/2*)
Alprazolam Xanax 6-12 hours
Amiodarone Pacerone 15-142 days
Atenolol Tenormin 6-7 hours
Clonazepam Klonopin 18-50 hours
Cocaine - 50 minutes
Diazepam Valium 20-100 hours
Donepezil Aricept 70 hours
Dutasteride Avodart 5 weeks
Erenumab Aimovig 28 days
Fluoxetine Prozac 2-4 days
Heroin - 2-6 minutes
Mercury - 65 days
Methamphetamine Desoxyn 6.4-15 hours
Methylphenidate Concerta, Ritalin 2-3 hours
Plutonium - 40 years (liver), 100 years (bone)
Phenytoin Dilantin, Phenytek 7-42 hours
Tetrahydrocannabinol (cannabis/marijuana) - Infrequent users: 1.3 days
Regular users: 13 days