Can someone please explain drug half life to me?

I thought i knew but i am getting confused. Let's say that Hydrocodone has a half life of 4 hours. If someone was to take 5mg would this mean that in 4 hours 2.5mg has been metabolized (active and effective) and in 8 hours 3.75mg has been effective? So after 8 hours there is still 1.25mg to metabolize or whatever? I read something about multiplying the half life by 5 to determine the length of effects. And how can you tell if a drug is fast acting? I thought that fast acting drugs would have shorter half life but i was wrong. Xanax has like a 10 hour half life and it is considered fast acting? What the hell? And oxycodone is 4 hours... Can someone explain this to me? Thank you.

Update:

I guess i want some sort of correlation between drug onset and half life.

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  • 9 years ago
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    A drug's half-life is determined by finding out how long it takes a medication or drug to be eliminated from blood plasma by one half of its strength. The drug's half-life is usually measured by looking at how medications degrade when they are administered via injection or intravenously in an IV drip. Theoretically, one dose of a drug administered by injection or through an intravenous (IV) line could be measured by looking at blood levels to see when the drug has reached half strength.

    There are some medications that have notoriously short half-lives, which means the body eliminates them and reduces them in strength very quickly. These include many over the counter pain relievers and medications that are used as tranquilizers like benzodiazepines. In fact, sometimes a short half-life can be a problem when you take things like Valium® or Xanax®. These may temporarily promote a sense of calm, but that feeling can wear off very quickly. This can create the need to take more of a drug than is really recommended.

    There is a specific formula to determine a drug's half-life, expressed as t(1/2) or time it takes to reduce strength by one half. Two important things that scientists look at to determine a drug’s half-life are volume of distribution and clearance. Clearance can be defined as the body’s removal of the medication from blood plasma. Volume of distribution refers to the way the drug gets distributed to other parts of the body.

    Another important term when considering a drug's half-life is bioavailability. This is the amount of a medication (or any substance) that is available for use in the body. When we take oral medications they must pass through our stomachs first, so they are less strong when they reach our circulatory system, and a short half life may mean we use very little of a drug if it is already near half strength when it gets to our bloodstream. Understanding the bioavailability of oral medications and a drug's half-life can help pharmacists and doctors determine appropriate dosage of medication.

    http://www.wisegeek.com/what-does-a-drugs-half-lif...

    http://en.wikipedia.org/wiki/Biological_half-life

    http://www.wisegeek.com/what-is-a-half-life.htm

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  • 3 years ago

    the assumption of the "a million/2 existence" may well be utilized to a radioactive decay, a chemical reaction or the breakdown of a drugs interior the physique. that's the time taken for a million/2 the unique cloth to decay/react/metabolise. If on the initiating we've a hundred%. Then after; a million a million/2 existence era 50% is left 2 a million/2 existence sessions 25% is left 3 a million/2 existence sessions 12.5% is left and so on. interior the above occasion after 48hrs a million/4 may well be left, after 72hrs a million/8,after 96hrs a million/sixteen and so on.

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