What is the half life of Adderall?

Your example is both a paradox and false. The ELIMINATION half life of Adderall is 10 hrs, ie., the amount in your body is not pharmaceutically significan after 10 hours. The half life is much less.

Contrary to the obviously misinformed, very pedantic, and unduly bitter commenter below, what you've described is exactly right. How he concludes that what you've written is a "paradox" is beyond me. The "elimination half-life" has nothing to do with whether the amount is "pharmaceudically significant" (as though there could be such an objective measure across all patients, another ridiculous claim). It simply means the time it takes for half of the substance to remain in your body. In fact, what he or she has said, is much more of a paradox. If half-life simply meant the time it took for the drug's effects to wear off, from where on earth would the word "half" come in? The ignorance is truly astounding.
Thomas Marley

The paradox is if you only lose half every X hours, then after 1000 hours you would have .5x10^3% left in your body. While it may work this way in the OP's mind, it doesn't in real life. The drug is gone (completely) in the first 24 hours.
Furthermore, I allowed the OP to retain some face with elimination half-life bit, but the truth is even the elimination half-life is much less than 10 hours. Elimination half-life is the time it takes for half of the pharmacologic effect to disappear. This differs from chemical half-life, the time it takes for half of the compound by volume to disappear. There is a distinction here because the elimination half-life effect is more useful for informing patients. The chemical half-life is often a logarithmic equation where the half-life is dependent on the initial concentration.

Summarized, the effect of adderall is gone in less than eight hours. The chemical is pretty much out of your body in full in 16.

The reason he states that it's a paradox is thus: If you take 10mg after 10 hours you will have 5mg, then 2.5, then 1.25, then .625 and so on but you will never reach 0. Not saying he's right, just explaining why that train of logic IS a paradox.

I know its been awhile since this was posted but I saw it & just had to comment. Suzanne66 is completely correct except our bodies are never technically "cleared" of the drug but are considered "clinically cleared" when enough of the drug is eliminated to pose no adverse affects.

All forgot to mention it is usually 4-5 half-lives before a substance is considered clinically cleared. So for Adderall that would be 40-50 hours.

If the half-life is 10 hours there would be no need for an extended release version.

Not only is the half life 10 hours which is correct in saying that half of the dose is still in the body 10 hours later, there is another point to note. The half life is 10 hours which we have established but that doesn't mean at 20 hours the initial dose is gone. So say you took 20mg at 12am at 10am you would have 10mg left circulating, at 8pm you would still have 5mg and 6am the next day you would have 2.5mg. I just thought it would be good to point this out as the meaning of half life seems to be misunderstood by one individual. Half life is simply how long it takes for half of the drug dose ( any drug for that matter ) to be excreted from you body. Of course extend relase and other drugs have different half lives but that wasnt the initial question. So what ever is left (ie the 20 then to 10 and the 10 to 5) will take 10 hours to be eliminated by half in your body.

Also I say the post about steady state and it is correct in saying it usually take about 4 to 5 doses to reach a steady state. Steady state is the point where a doctor would look for pharmacotherapuetic effects. Is it working for the patient and creating the desired effect? Slow release or controlled release help a person receive a dose over an extended period of time without having to take multiple pills because the body hasn't had a chance to degraded it. Look up hepatic first pass. All drugs taken orally pass through the liver and are degraded. Slow release/ extend release allows you to keep a more consistent dose through out the day

Old post, but... to be exact, it takes 3.3 half-lives to reach steady state and 5 half-lives to be cleared from a persons body. These values are used when using calculations regarding pharmacokinetics.