Psycho-Babble Medication Thread 577993

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Half Live vs. Duration of Action

Posted by mike99 on November 12, 2005, at 9:16:32


Can anyone explain how dextroamphetamine has a plasma half-life of approximately 10 hours yet it's duration of action is only 4-6 hours?

For example, if one took 10 mg, wouldn't you then have the effect of 5 mg ten hours later?

I'm also curious how accumulation does not occur with a half-life over 5-6 hours or multiple daily dosings as my understanding is that 4-5 half-lives are required to eliminate a drug from the body.

Any thoughts on this?

 

Re: Half Live vs. Duration of Action mike99

Posted by Kon on November 12, 2005, at 18:45:03

In reply to Half Live vs. Duration of Action, posted by mike99 on November 12, 2005, at 9:16:32

> Can anyone explain how dextroamphetamine has a plasma half-life of approximately 10 hours yet it's duration of action is only 4-6 hours?

Because of redistribution of drug to other sites in the body other than central (brain) receptors.

> For example, if one took 10 mg, wouldn't you then have the effect of 5 mg ten hours later?

No...for same reason. A 5 mg immediate dose means a greater % of total drug hitting the central receptors because the drug hasn't fully distributed to the whole body so more can hit the brain receptors. With a 5 mg dose left over after 10 hours (half-life), a greater percentage of drug will be distributed to other body sites other than the brain. So while the same amount of drug is in body (5 mg), there will be less drug hitting brain receptors in the latter situation.

> I'm also curious how accumulation does not occur with a half-life over 5-6 hours or multiple daily dosings as my understanding is that 4-5 half-lives are required to eliminate a drug from the body.

There may be accumulation to some steady state blood level depending on dose, dosing interval and half-life.

 

Re: Half Live vs. Duration of Action

Posted by mike99 on November 13, 2005, at 14:42:51

In reply to Re: Half Live vs. Duration of Action mike99, posted by Kon on November 12, 2005, at 18:45:03


Thanks for the information Kon. You explained it very well.


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