The effectiveness of a specific medication relies on there being adequate levels of the drug in the body to give a therapeutic effect. Every drug degrades at a different rate. A drug's half-life, which is a measure of how quickly or slowly it degrades, will determine how frequently the drug needs to be taken to maintain its therapeutic effect. The duration of action of a drug is also determined by its half-life, and therefore the period of time between dosages depends on it.
A drug's half-life is essentially the time it takes for it to degrade to half of its initial amount. This is made slightly more complicated by the fact that there is a biological half-life and a plasma half-life. The biological half-life, otherwise known as the elimination half-life, refers to the amount of time it takes to reach half of the initial activity of the drug. Plasma half-life refers to the time taken purely for there to be half the amount of drug present in the blood.
There are many factors that influence the action of a drug on the body, not only a drug's half-life. Drugs may be metabolized by various mechanisms, including liver enzymes, renal metabolism and metabolism by other enzymes. The drug may also be stored in tissues or fats, prolonging that drug's action. Despite all of these contributing factors, however, the half-life is a relatively good predictor of the dosage regimen of that drug.
Should a drug's half-life be very short, it would mean that the dosage would be given more frequently, to keep the levels in the body up and have a consistent therapeutic effect. For example, with analgesia, the prescribing doctor will give a dosage and schedule that will prevent breakthrough pain. Depending on the drug's half-life, this may mean dosing once a day or three to four times a day. The dosage recommended by the prescribing doctor should not be exceeded, as the levels of the drug may then exceed therapeutic levels and result in overdose.
Some substances, such as the bisphosphonates, which are used in the treatment of osteoporosis, have a very long half-life and need only be taken weekly, monthly, or even yearly. Other drugs, such as some antibiotics, may have a shorter half-life and require a minimum concentration to be effective, so dosage may be three or four times daily. The full course of antibiotics should always be completed, and dosage instructions from the prescribing doctor followed precisely.