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A drug’s half-life is the time it takes for half of a given dose to be eliminated from the body or bloodstream. This value varies considerably between different types of medications, and even different preparations of the same one. The most influential factor is the way the drug has been chemically prepared, but variables such as a person’s health and metabolism can also effect how long it takes to pass from the body. Medical professionals who prescribe and dispense drugs use all of this information when deciding on the best drug for a patient.
Biological and Plasma Half-Life
There are two ways to quantify the half-life of a medication. The biological or elimination half-life is the time it takes for the bioactivity of the drug to reduce by 50% of its initial value. In contrast, the plasma half-life is the time it takes for the concentration of the drug in the bloodstream to reduce by 50%.
Two related terms are clearance and volume of distribution. Clearance refers to the speed at which the drug is eliminated from blood plasma, while volume of distribution is a measurement of the amount of drug distributed throughout the body’s tissues. Half-life, clearance, and volume of distribution are important measurements for assessing how effective a given drug regimen is, particularly when a new drug is being developed and tested.
Factors that Affect How Long a Drug Lasts
Many things can influence a drug’s clearance rate and half-life. One is a person’s metabolism, which can change the rate of clearance from the bloodstream and affect the speed at which a drug is processed. Liver and kidney health are also important, as the liver is a site of drug metabolism, and the kidneys are crucial in blood filtration. The medications a person is taking can interact with one another to modify the half-life of any of them as well. Many medications for the treatment of thyroid disorders and some antipsychotics, for example, can have this effect. Even basic factors such as age and sex can play a role, as they affect metabolic rate.
The effects of a drug typically begin to taper off as the first half-life point is reached. In most cases, 90% to 95% of a drug is eliminated after four cycles. For instance, if a drug’s half-life is ten hours, it will take 40 hours for approximately 95% of the drug to be eliminated from the blood plasma. This is important information for people who are taking prescription medications that are regularly screened for in blood tests. Someone who is taking prescription steroids for conditions such as acne or asthma might test positive for steroids in an employment-related drug test, for example.
Drugs with Short Half-Lives
Medications in this category must be taken several times a day, to keep the concentration of the drug high enough for it to be effective. Many over-the-counter and prescription painkillers, as well as many types of tranquilizers, have short half-lives. Other medications that are cleared from the bloodstream quickly include antibiotics and insulin.
Drugs such as tranquilizers and pain relievers are often addictive, particularly when they have very short half-lives. This is because their effects are fast and strong, but because they wear off quickly, the patient needs to take more to continue feeling relief. When these types of medications must be taken in the long term, a medical professional will typically try to prescribe those that remain active for longer to reduce the possibility that dependency might develop.
Drugs with Long Half-Lives
Medications that are eliminated from blood plasma more slowly remain at effective doses for much longer periods of time, from days to weeks to months, and even longer. For example, biophosphonates, which are used to treat osteoporosis, are absorbed by bones and have half-lives that can extend for many years. It is much more common, however, for a drug’s half-life to be measured in terms of hours and days.
The longer a drug’s half-life is, the longer it takes for the body to eliminate a single dose. This is medically significant in many instances, because drugs can interact with one another, and because some can influence the success of a surgical operation. For example, the blood thinner warfarin is prescribed to people who are at risk of developing blood clots. A person who is preparing to undergo a surgical procedure must stop taking the drug several days in advance, to eliminate it from the body and reduce the risk of excessive bleeding during the operation.
Base drugs can be prepared in different ways to yield products that are effective for different lengths of time. For example, morphine has a half-life of just two hours, but the derivatives codeine and tramadol last for four and six hours, respectively. Access to drug variants allows medical professionals to treat pain with a high degree of specificity.
Acute pain, such as that experienced during a gallbladder attack, is often treated with morphine. A person who is recovering from gallbladder removal surgery, on the other hand, has pain of lower intensity, but that pain might linger for several weeks. He or she is therefore more likely to be prescribed codeine or another derivative with a longer half-life and a lower risk of dependence.