How many half lives until steady state

And imagine that when you use 2 eggs to make an omelette for breakfast. Someone in your house notices the empty spots in the carton of eggs and purchases 2 more eggs and places them in the carton. So when you wake up the next morning, the carton is full of eggs. If this process repeats itself over many days, it would appear that the eggs never change … there are always 12 eggs in the carton even though you use them for various meals and recipes.

In this hypothetical scenario, the eggs are at steady state because the rate of elimination is equal to the rate of input. The eggs represent individual drug molecules in the body. Using the eggs represents the variety of clearance mechanisms that eliminate drug molecules from circulation. And the replenishment of eggs represent taking new doses of medication. There are several special PK parameters associated with steady state kinetics.

These parameters are not necessarily more important; however, they are useful because of the unique situation when drug input rate and elimination rate are equivalent. The first is the average plasma concentration at steady state, or C ss. This parameter can be calculated based on the steady state definition where the rate of input is equal to the rate of elimination. Rate of elimination:. By solving for C p , you get the following:.

As a further simplification, we know that there is a relationship between dose, clearance, and bioavailability shown by the following equation:. Thus, the average concentration at steady state is simply the total exposure over 1 dosing interval divided by the time of the dosing interval. So while concentrations rise and fall during a dosing interval at steady state, the average concentration does not change. Furthermore, the only factors that control C ss are the dose, the dosing interval, and the clearance.

Do you have more questions about how steady-state concentration is used in drug studies? Contact us today to speak with one of our senior consultants. Download Blog as PDF. Half-life and Steady-State Concentration A half-life is how long it takes for half of the drug to be eliminated from the body. Another way to think about steady state: After Dose 1: There are 0. After Dose 2: There are 1. After Dose 3: There are 1.

After Dose 4: There are 1. After Dose 5: There are 1. Loading Dose and Steady-State Concentration For a drug with a short half-life, steady state is achieved pretty quickly. Factors That Affect Steady-State Concentration Steady-state concentration can fluctuate depending on many factors, such as: Drug clearance Dosing interval Dose Drug clearance Drug clearance CL dictates the rate at which a drug is eliminated from the body.

Dosing interval The dosing interval affects steady-state concentration in a proportional way. Dose In the same way, higher doses will increase the C ss values, and lower doses will decrease them. Importance of Steady State in Drug Development In studies conducted in special populations, and in studies for assessing drug interactions, you might be required to take any necessary measurements when drug concentrations have reached steady state.

To be effective the drug must reach a certain level and so must our water in the bucket but the body is not a closed system and the drug can be lost by being broken down in the liver and excreted from the kidneys. This loss of drug from the body can be represented by putting a small hole in the bucket so that some water is constantly leaking out. Like the drug level in the body, the level of water drops and needs to be topped up by giving regular doses.