Transcript 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.