Every medication is unique in terms of how long it stays in the body. The term “half-life” is used describe the length of time it takes the body to clear/eliminate/metabolize 50 percent of a dose.
This concept leads to the goal of a “steady state,” which is where the daily dose equals what the body eliminates over the course of a day. Halflife is also important to take into consideration when stopping a medication, because it takes roughly five half-lives to eliminate more than 90 percent of a medication from the body.
The half life of a medication can be very brief, on the order of minutes as is often seen in anesthesia, or it can be days and even weeks. The antidepressant fluoxetine (Prozac) is a good example, as it has a half-life on the order of seven to ten days. This is neither a good nor a bad thing, but it is important to know.
So if someone needed to be off Prozac and have it out of her system (five half-lives) it would take over a month, possibly longer in an older person. On the flip side, you can see that if someone has been on Prozac for a long time and is at a steady state, missing a dose or two will have little effect on her blood level.
The half-life of a medication can be seriously shortened or lengthened when someone is taking another medication that impacts its elimination from the body. For people with bipolar disorder this becomes especially pertinent with mood stabilizers such as lithium, valproate (Depakote), and other medications whose half-lives can be significantly altered (increased or decreased) when on multi-drug regimens.
The result is that levels of a medication can become either dangerously high or too low to work. This is one of the reasons it’s important to maintain an accurate list of medications that can be reviewed anytime a drug is being added, adjusted, or discontinued (an index card in your wallet or pocketbook works well).