Once a drug is absorbed it is distributed through the body, after which it begins to be eliminated. The concentration of the drug in the plasma reflects the amount of drug in the body at the time.
Route of Administration
Rate of Absorption
Plasma Concentration Peak
intravenous, bolus
NA
very soon after dose, maximum possible
oral
rapid
soon after dose, may be similar to IV bolus dose
oral
slow
delayed, may be much less than IV bolus dose
Plasma drug concentrations after oral or intravenous dosing show the same rate of decline from the same peak concentration.
After an intravenous (IV) bolus:
plasma drug concentration at time T =
= ((plasma drug concentration at time=0) * (fraction of drug remaining in the body)) =
= ((plasma drug concentration at time=0) * (EXP((-1) * (elimination rate constant) * (T))))
where:
• "time=0" is immediately after injecting the bolus
dose of drug = amount of drug in the body at time 0 =
= (plasma drug concentration at T = 0) * (volume of distribution)
(amount of drug left in the body at time T) =
= (plasma drug concentration at time T) * (volume of distribution))
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