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Steady state is reached after about 5 × 12 = 60 hours. Pharmacokinetics (from Ancient Greek pharmakon "drug" and kinetikos "moving, putting in motion"; see chemical kinetics), sometimes abbreviated as PK, is a branch of pharmacology dedicated to describing how the body affects a specific substance after administration. [1]
For this reason, when a drug is introduced into the body at a constant rate by intravenous therapy, it approaches a new steady concentration in the blood at a rate defined by its half-life. Similarly, when the intravenous infusion is ended, the drug concentration decreases exponentially and reaches an undetectable level after 5–6 half-lives ...
A physiologic interpretation of clearance (at steady-state) is that clearance is a ratio of the mass generation and blood (or plasma) concentration. Its definition follows from the differential equation that describes exponential decay and is used to model kidney function and hemodialysis machine function:
This is not to be confused with dose regimen, which is a type of drug therapy in which the dose [mg] of a drug is given at a regular dosing interval on a repetitive basis. Continuing the maintenance dose for about 4 to 5 half-lives (t 1/2) of the drug will approximate the steady state level. [1]
In pharmacokinetics, the drug accumulation ratio (R ac) is the ratio of accumulation of a drug under steady state conditions (i.e., after repeated administration) as compared to a single dose. The higher the value, the more the drug accumulates in the body. An R ac of 1 means no accumulation.
In clinical practice, this means that it takes 4 to 5 times the half-life for a drug's serum concentration to reach steady state after regular dosing is started, stopped, or the dose changed. So, for example, digoxin has a half-life (or t 1 / 2 ) of 24–36 h; this means that a change in the dose will take the best part of a week to ...
The steady state or stable concentration is reached when the drug's supply to the blood plasma is the same as the rate of elimination from the plasma. It is necessary to calculate this concentration in order to decide the period between doses and the amount of drug supplied with each dose in prolonged treatments.
At steady state, the concentration of free drug in the central compartment (i.e. circulation system) is equal to the concentration of free drug in the peripheral compartment (i.e. body tissues) If steady state is reached, context-sensitive half-life is equal to elimination half-life Only free drug that is in the plasma is metabolised