Download Half-Life

Objective: Explain the variation in
dosing intervals for medications
1)
2)
•
•
•
Introduce Half-Life (t1/2)
Relate Half-Life to
Dosing intervals of aspirin and Aleve pain relievers
Longevity of a Single Dose
Time to Therapeutic Steady State
Half-Life-t1/2
The time it takes a substance to decrease
to half of a previous value.
2H2O2
O2 + 2H2O
Hydrogen Peroxide
t1/2 = 654 minutes, appx 11 hours
√
Hydrogen Peroxide Half-Life
Half-Life is a good predictor of how long a substance will exist
50%>
25%>
12.5%>
1
2
3
4
<6.25%
Drug Half-Lives
The time required for the total amount of a drug in blood plasma to
diminish by half, once the drug reaches its peak concentration.
Peak Amounts
Time to Peak: 1.5hrs
Dosing Intervals Half-Life
Aspirin
Aleve
Half-Life (PDR)
6 hours
12-17 hours
Dosage Interval
4 Hours
12 Hours
In general, next dose is given once the previous dose reaches its half-life
Normal Dose Interval is based on one Half-Life
PDR: Physicians Desk Reference
√
Draw Half-Life curves for Aspirin and Aleve
Estimate the Amount and Percent of Drug remaining after
two Half-Lives
Half-Life (PDR) Time to Peak (PDR)
Aspirin
6 hours
1-2 hours
Aleve
12-17 hours
1-2 hours
50%
25%
50%
25%
Why do Drugs Have Different Half-Lives?
Sodium
O- Na+
Pharmacokinetics
•Absorption
•Distribution
•Metabolism
•Excretion
Molecular Structure
Bond Polarity
√
Half-Life and
Longevity of a Single Dose
Rule of thumb: For a single dose, a drug is
eliminated from the body after five half-lives.
Half-Life
(PDR)
Elimination
time
Aspirin
6 hours
30 hours
Aleve
12 hours
60 hours
Half-Life and Therapeutic
Steady State Level
Rule of thumb: It takes five dose intervals for a
drug to reach a therapeutic steady state level.
About How Many Half-Lives?
5
√
Q: What if a drug has too long of
a half life to relieve a condition?
A: A loading dose is
administered followed by
maintenance doses
Examples of Drugs administered with
Loading + Maintenance doses
Digoxin: Treat arrhythmia
t1/2 = 1.6 days.
8 days or five half lives.
To reach therapeutic steady state would take __?__
Medrol: Steroid to reduce inflammation
How much time would pass to attain a therapeutic steady state
amount?
t1/2 = 18-26 hours
About 5 days
What to Know…
 Half-Life: The time it takes a substance to decrease to half of a previous value.
 Half-Life is a good predictor of how long a substance will exist.
 Drug Half-Life: The time required for the total amount of a drug in blood to
diminish by half, once the drug reaches its peak concentration. See PDR
 How to construct a half-life curve for a drug.
 A dose interval is approximately one Half-Life.
 Pharmacokinetics is the study of reaction rates of drugs in biological systems.
 Rule of thumb: For a single dose, a drug is eliminated from the body after
five half-lives.
 Rule of thumb: It takes five dose intervals or five half-lives for a drug to
reach a therapeutic steady state amount.