Use and management of medications for children diagnosed with Attention Deficit Hyperactivity Disorder (ADHD)

Focus on Exceptional Children, Jan 2001 by Kollins, Scott H, Barkley, Russell A, DuPaul, George J

The rate at which different drugs are broken down and eliminated from the body is described generally as the halflife of a medication. This term is used to describe the time required to eliminate half of a drug from the body (hence the term half-life). For example, the half-life of normally administered methylphenidate (Ritalin) is 2 to 3 hours (Julien, 1998). This means that it takes between 2 and 3 hours for half an initial dose to be eliminated from the body. This also means that 4 to 6 hours after taking a dose of methylphenidate, only 25% is left in the body-50%, or half of the drug, is left after the first 2-3 hours, and 50% of 50% (25%) is left after another 2-3 hours.

The half-life of other drugs can be much longer. For example, fluoxetine (Prozac), an antidepressant drug that has been used to treat ADHD has a half-life of 1 to 4 days. This means that, especially if the medication is administered daily, it takes much longer for the drug to be eliminated completely from the body. Conversely, it also means that it may take several days (or weeks) for a drug to reach "therapeutic levels" in the body.

For example, suppose that a drug has a half-life of 1 day (24 hours) and a dose of 10-mg is prescribed once daily. On the second day, 5 mg is left in the body, and this is added to that day's additional 10 mg dose. On the third day, 2.5 mg of the original dose, plus 5 mg of the second dose, plus 10 mg of that day's dose is left in the body, for a total of 17.5 mg. This regimen may be continued until reaching a certain level of the drug in the body, at which time the dosing regimen may be altered to maintain this level.

One might ask why not just administer a larger dose all at once? In general, this might not be possible with some drugs because of potentially problematic side effects of large doses given in one administration. In any case, an understanding of the concept of half-life of drugs is important to help teachers and parents understand the dosing regimen on which their child may be placed. Further, this information might be helpful in determining whether changes or aberrations in behavior or other functioning of a child are likely the result of drug effects or other factors.

The half-life of a given medication typically varies somewhat across individuals. For example, the average half-life of methylphenidate (Ritalin) is between 2 and 3 hours. For a child in whom Ritalin has a half-life of 2 hours, only 25% of the medication would remain 4 hours after a dose. By contrast, a child in whom Ritalin has a half-life of 3 hours would not eliminate that much of the medication until 6 hours after a dose. These differences may require different dosing regimens for different children to attain maximum clinical efficacy. The former child may benefit from doses administered every 4 hours, whereas the latter child may benefit from a more delayed regimen. Of course, as noted previously, the best way to determine the best dosing schedule is to conduct an individualized medication assessment.