half-life
The half-life is the time it takes for the amount of drug in your blood to drop to half of what it was. If a drug has a half-life of four hours, then after four hours half is gone, after eight hours three-quarters is gone, after twelve hours seven-eighths is gone, and so on — each fixed interval clears away half of whatever remains.
This steady halving is the signature of first-order elimination, where the body removes a constant fraction of the drug per unit time rather than a fixed amount. The practical consequence is a rule of thumb: after about four to five half-lives, a drug is essentially gone (around 94 to 97 percent cleared), and conversely it takes about the same number of half-lives of regular dosing to build up to steady state.
Half-life is not a fundamental property in its own right — it is derived from clearance and volume of distribution. A drug can have a long half-life because the body clears it slowly, or because it hides deep in the tissues (large volume of distribution), or both. Half-life mainly governs how often a drug must be dosed and how long its effects linger.
A drug with a 24-hour half-life suits once-daily dosing and reaches steady state after roughly four to five days, whereas a drug with a 1-hour half-life may need frequent dosing or a slow-release formulation to stay effective.
Half-life sets the dosing interval and the time to steady state.
Half-life equals 0.693 times the volume of distribution divided by clearance. So a longer half-life can come from slower clearance OR a larger volume of distribution — the number alone does not tell you which.