The half-life of a drug is the time it takes for the amount of a drugs active substance in your body to reduce by half. This depends on how the body processes and gets rid of the drug, and it can vary from a few hours to a few days, or sometimes weeks. No matter what dosage of a drug youre on, or how long youve been taking it for, its half-life is always the same. The half-life of a drug is an estimate of the time it takes for the concentration or amount in the body of that drug to be reduced by exactly one-half (50%) . The symbol for half-life is t½. Different drugs have different half-lives. The actual half-life of a drug varies from person to person, depending on a number of different patient- and drug-specific factors. The half-life can be a guide for how long a drug will take to reach a stable level in your body when you first start taking it. Generally, it will take about five times the drugs half-life to build a stable level in your body. Once a drugs level is stable in your body, any early side effects you experience from the drug may start to decrease. The half-life of a drug matters because a short half-life usually means more withdrawal problems, while a long half-life usually means fewer withdrawal problems. If you are taking a drug with a short half-life and having problems with withdrawal, it might be possible for you to switch to a similar drug with a longer half-life.