Drug Half-life Explained
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 |
Amphetamine | Adderall, Dexedrine | 10-12 hours |
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 |
Lead | - | 28-36 days |
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 |
Warfarin | Coumadin, Jantoven | 1 week |
* 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
- Medical Conversions - How many mL in a teaspoon?
- Pill splitting - Is it safe?
- 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?
Sources
- What is a drug's half-life? Mind UK. https://www.mind.org.uk/information-support/drugs-and-treatments/medication/explaining-the-half-life/#:~:text=What%20is%20a%20drug's%20half,few%20days%2C%20or%20sometimes%20weeks.
- Hallare J, Gerriets V. Half Life. [Updated 2021 Aug 23]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK554498/
- Pharmacology: Half-life of Drugs School of Health Sciences. The University of Nottingham. https://www.nottingham.ac.uk/nmp/sonet/rlos/bioproc/halflife/index.html
Further information
Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.