Every medication has a certain amount of time that it remains "active" before the body begins to metabolize or excrete the medication. The amount of time it lasts in the body before it is half gone is called the half life.
The half life is an important concept for clinicians;
- When treating infections, the antibiotic needs to be present long enough and at a high enough dose to kill the infection
- When taking a sleeping medication, you want it to work only so long that you wake up refreshed, not groggy
- When being treated for blood pressure problems (Hypertension) or cholesterol problems (Hyperlipidemia) you'd like the medication to work all day.
Some medical conditions will either shorten or prolong the half life of a medication, which is important for the clinician to consider, to avoid over or under dosing.
- Kidney disease will reduce the excretion of medications that are primarily eliminated through the kidney, allowing it to increase in the blood. This is predictable and should be taken into consideration by the clinician
- Certain medications interfere with the metabolism of other medications, either by speeding up, or slowing down, the metabolism. The clinician has the information to know what does what to what, to avoid this problem.
- Certain foods can interfere with metabolism of medication. Grapefruit and grapefruit juice can cause problems with this, and the clinician may tell one to avoid this food.
Mathematically and biologically, it takes four half lifes for a medication to reach steady state, or put another way, a consistent level in the body. For a medication that has a 4 hour half life, it will take 4 doses every four hours to reach the correct level. For this reason, sometimes clinicians advise "to take two the first time", which reduces the amount of time before reaching a steady state.
Also, when a medication has to be stopped, it is often desirable to reduce it over time, instead of stopping it all at once. This may reduce the uncomfortable effects of stopping it. Examples include:
- Narcotic addiction: stopping the narcotics all at once may bring on serious and severe side effects, whereas reducing them a little at a time is more tolerable and comfortable.
- Paxil (paroxetine), like SSRIs generally, is a difficult medication to stop all at once, and should be tapered over time
- Prednisone, and other steroids, may need to be tapered for a different reason...their presence in the body will allow the adrenal glands to "go on vacation", and stopping them suddenly after several days may cause serious problems, whereas slowing them down will allow the adrenal glands to "recover" from the vacation.