HALF-LIFE
The half-life of a drug is the amount of time it takes for half of the active ingredient in a drug to be eliminated from the body. If you take the example of a drug that has a half-life of five hours, this means that half of the active ingredient of the drug will be eliminated from the body in five hours. (Of course this is very dependent on an individual’s DNA and the metabolism of the drug.)

LENGTH OF TIME IN THE BODY
Generally, the elimination of the other half of the active ingredient of the drug takes much longer. In fact, for most drugs it is estimated that in the next five hours our sample drug (with a five hour half-life) would have eliminated about half of the remaining half, so one- quarter of the drug would still remain in the body. In the next five hours, another half of the remaining drug would be eliminated and this would mean that one-eighth of the drug remained in the body. In the next five hours, another half of the remaining drug would be eliminated and 1/16 of the drug would remain in the body. Then in the next five hours another half of the remaining drug would be eliminated and 1/32 of the drug would remain in the body.

It is considered that when all but 1/32 of a drug has been eliminated, the concentration of the drug’s active ingredient will no longer be able to affect the body. This is why it is assumed that a drug is gone from the body after five half-lives even though a small portion remains.

In fact, there is increasing medical evidence that some portion of many drugs will be absorbed into the body’s fatty tissues and remain there until discharged when the fatty tissues are broken down. This may explain why people who have taken drugs for a long time may find strong cravings for the drugs when they go on a diet. The drug that was trapped in the fatty tissues is released into the bloodstream and goes to the brain, and the brain sees the chemical is back and looks for more.

STEADY STATE OR STEADY CONCENTRATION
This refers to a situation where a drug is not taken once but is taken regularly and the actual concentration in the body stays within a range and does not vary widely. It is estimated that to achieve a steady state, most drugs will need to be administered for a length of time equal to five half-lives of the drug.