Clinical significance of half-life
Example:
A 70 year-old man is on gentamicin for urinary sepsis and is found to have a toxic blood level of 16mg/L
- The blood level you want is 2mg/L
- Drug half-life is 8 hours
Question: How long will it take for the blood level to fall back to the level you want?
Answer
Remember Half-Life is time taken for blood level to reduce by 50%
Therefore:
16mg to 8mg = 8 hours
8mg to 4mg = 8 hours
4mg to 2mg = 8 hours
Total = 3 x 8 hrs = 24 hours
Therapeutic index = a ratio that compares the dose of a drug that causes the therapeutic effect to the dose that causes toxicity
Digoxin
Gentamicin
Carbamazepine
Phenytoin
Lithium
theophylline
Examples of drugs where plasma concentration is measured
Phenytoin: to ensure suppression of seizures
Gentamicin: to prevent toxicity
So how do you change the plasma concentration?
To change the plasma concentration either:
Change the dose or
Change the dosage interval
So it’s better to give dihydrocodeine as 30mg every 4 hours rather than 60mg every 8.