Objective: Explain the variation in

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Objective Explain the variation in dosing
intervals for medications

• 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

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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
v
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Hydrogen Peroxide Half-Life
Half-Life is a good predictor of how long a
substance will exist
50gt
25gt
12.5gt
lt6.25
2
4
3
1
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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
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Dosing Intervals Half-Life
  Half-Life (PDR) Dosage Interval
Aspirin 6 hours 4 Hours
Aleve 12-17 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
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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
7
50
25
8
50
25
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Why do Drugs Have Different Half-Lives?

• Pharmacokinetics
• Absorption
• Distribution
• Metabolism
• Excretion

O- Na
Sodium
Molecular Structure Bond Polarity
v
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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
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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.

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About How Many Half-Lives?
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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
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Examples of Drugs administered with Loading
Maintenance doses
Digoxin Treat arrhythmia t1/2 1.6 days.
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To reach therapeutic steady state would take
__?__ days or five half lives.
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
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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.