# Drug Half-life Explained

Written by C. Fookes, BPharm on Mar 8, 2019

## 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 period of time that 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 variables. 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 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 sample of choice in 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 (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 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