Question

A patient will start on an anti-seizure drug that has a half-life of up to 42 hours. When will the patient achieve steady-state? Between days and days. (Answer to the nearest whole numbers.)

Answer 1

In pharmacokinetics, a drug is said to have attained a steady state when the rate of intake of the drug equals the rate of elimination. This is a state of dynamic equilibrium where the plasma concentration of the drug remains constantly in the therapeutic level. This concept is especially important for medicines that are administered chronically.

Time taken for attaining steady state is determined in terms of the elimination t½ of the drug. t½ is the time taken for the drug to reach half its initial concentration. So, in one t½, half of the drug will be eliminated and only 50% will remain in the plasma. In other words it is said to have attained 50% steady state.

Usually it takes 5 t½ for a drug to attain steady state of 97% and this time is considered as time for attaining steady state. This can be shortened by giving a loading dose.

In our question, loading dose is not mentioned. Hence the time taken to attain steady state for the antiseizure medication = 5 times t½= 5 X 42 hours = 210 hours.

This will fall between day 9 and 10 since initiation of therapy, considering the day of initiation of the drug as day 1.