With fluoxetine it has a half-life of 5 - 7 days, yet patients are told to take it every day. This would seem to indicate that the plasma concentration will just continue to increase, especially after steady-state concentration around days 25 - 35.
It wouldn't. It reaches steady state when the rate of clearance equals the dose. The more in your system as it builds up, the more your body clears. Hence it's half life as in half the drug regardless of the plasma concentration. Once it builds up to a level where the body clears the amount in 24 hrs to how much your dosing every 24 hrs, it reaches steady state. For this drug it's about 30 days. You are imagining a drug that is eliminated via zero order kinetics, where the elimination rate is constant regardless of plasma concentration.
@@emphor88 Sure. It depends what you mean by "much" but lets assume its 10x shorter than the dosing interval. Technically, from google, "a steady-state is reached when the quantity of drug eliminated in the unit of time equals the quantity of the drug that reaches the systemic circulation in the unit of time." So if you are dosing a drug every 5 days that has a half life of 12 hours, it is virtually all eliminated from your system when you dose again. If you define the "unit of time" as 5 days, then I guess you would be at steady state theoretically, as you are clearing as much drug in 5 days as you are dosing in 5 days.
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With fluoxetine it has a half-life of 5 - 7 days, yet patients are told to take it every day. This would seem to indicate that the plasma concentration will just continue to increase, especially after steady-state concentration around days 25 - 35.
It wouldn't. It reaches steady state when the rate of clearance equals the dose. The more in your system as it builds up, the more your body clears. Hence it's half life as in half the drug regardless of the plasma concentration. Once it builds up to a level where the body clears the amount in 24 hrs to how much your dosing every 24 hrs, it reaches steady state. For this drug it's about 30 days. You are imagining a drug that is eliminated via zero order kinetics, where the elimination rate is constant regardless of plasma concentration.
If your at "steady state" you can't be increasing your average plasma level
As you can see at 7:35, even when the half life is longer than the dosing interval, it still reaches steady state.
@@davidbarry2837 What if the half-life is much shorter than the dosing interval? Would you ever reach steady state?
@@emphor88 Sure. It depends what you mean by "much" but lets assume its 10x shorter than the dosing interval. Technically, from google, "a steady-state is reached when the quantity of drug eliminated in the unit of time equals the quantity of the drug that reaches the systemic circulation in the unit of time." So if you are dosing a drug every 5 days that has a half life of 12 hours, it is virtually all eliminated from your system when you dose again. If you define the "unit of time" as 5 days, then I guess you would be at steady state theoretically, as you are clearing as much drug in 5 days as you are dosing in 5 days.
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I like the video except for the hokey music. It’s difficult enough to Cross by your contacts without having that silly soundtrack in the background.
Please remove the music , it can't concentrate on music and your video
yeah EXTREMELY good video but VERY annoying music