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How Is Dosage Affected by a Drug's Half-Life?

By B. Chisholm
Updated Mar 03, 2024
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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.

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Discussion Comments
By anon1003203 — On May 21, 2020

Charred,

"I’ve heard of people on the Internet advocating that people can pass certain urine tests by knowing the different half lives of various drugs and scheduling the test after the drugs have more or less disappeared from the bloodstream."

The half life of a drug can estimate the time for complete elimination. So if a drug of 30 mg has a half life of 10 hours, then in 10 hours you have 15 mg. In 10 more hours, you have 7.5 mg. In 10 more hours, you have 3.75. In 10 more hours, you have 1.875. 10 more hours, you have 0.93, 10 more, 0.46. 10 more, 0.23, etc If they are testing for a drug, like above (depending on what the cutoff amount is for what they consider to be a positive result) you may wait a few days to test. However, marijuana, for instance, is like 14 days, I think(?) I don't know, but lets assume it is. So if you had an amount of marijuana that was comparable to the amount of the drug above, and you wanted the amount to be less than 1, then you would have to wait 5 half lives, which would be 70 days.

Also, elimination of the drug means that the actual drug is gone, and would result in a test result that is negative FOR THE DRUG. However, they may not be testing for the drug, rather they may be testing for the metabolites that your body produces as it processes the drug, and this is very different. Alcohol is metabolized at 1 drink per hour. (It is linear, and is different than a half life.) Long after it is eliminated, they can test for the metabolites (ETG). Those hang around up to 80 hours.

"They also advocate eating certain kinds of foods like poppy seeds, which are supposedly expected to dilute the presence of the drug in the bloodstream."

Poppy seeds will give a false positive for opiates. I thought this was something that could occur only in the most extreme circumstances, but I have known 2 people who this has happened to, and I really don't know that many people. One was a school teacher and the other an auto mechanic. Neither had done a drug in their life. They neither smoked nor drank. They both discontinued eating the poppy seeds, retested, and the tests were clean. I can only imagine that purposefully eating the seeds would be to have an explanation for the positive test result, and possibly buy them time to get clean? Not sure.

By NathanG — On Aug 12, 2011

@hamje32 - Yes, they’re accurate, but they have to be used right away.

Alcohol doesn’t stay in the body forever. Basically when alcohol is consumed some of it evaporates and permeates your lungs; hence, it winds up on your breath.

The breath device that the police use actually measures the amount of alcohol on your breath. Of course, if it’s high, the officer will probably smell it anyway.

But the device will give them a certain number. This figure maps to a comparable amount in your bloodstream from what I understand.

In other words, they can use that number and extrapolate from it how much alcohol would be in your blood, were they to do a blood test on you. Of course, the breath test is much more convenient and delivers instant results.

By hamje32 — On Aug 11, 2011

How does alcohol testing with breath devices work? I hear that they’re pretty accurate.

By miriam98 — On Aug 11, 2011

@Charred - I say that much of it is lore. When doctors conduct urine tests, they are not just measuring for the presence of the drug itself. They are measuring for its byproducts.

Once the drug enters your body it can become metabolized, and these are the byproducts that the tests measure for. In that sense, it’s my understanding that the tests can detect the presence of the drug long after its half life has expired.

I think for marijuana, for example, they can detect it anywhere from 10 days to 30 days, depending on how much marijuana was smoked.

I agree with you however; it’s better to say no to drugs than to try to figure out ways to cheat the system. People who do drugs will have more problems on their hands than whether or not they passed urine tests.

By Charred — On Aug 10, 2011

What does half life have to do with urine tests?

I’ve heard of people on the Internet advocating that people can pass certain urine tests by knowing the different half lives of various drugs and scheduling the test after the drugs have more or less disappeared from the bloodstream.

They also advocate eating certain kinds of foods like poppy seeds, which are supposedly expected to dilute the presence of the drug in the bloodstream.

I am not advocating illegal drugs in any way; I just want to know if these claims are scientifically credible.

By Sinbad — On Aug 10, 2011

I have found in talking with friends who like me have a family member with mental disorders that may need to try a variety of medications and a variety of possible dosages of the medication; that the knowledge of the half-life of the pill is important.

For me and helping my family members what is best for them, it is important to analyze the half-life of the pill to know if it has been given long enough time to be therapeutic to the patient as well as also knowing how long a person may need to try a pill before it becomes regulated.

For example, one of my family members is on a medication for obsessive compulsive disorder and the articles we read about the medications let us know that it would be weeks before any answers for the medications true effect could be determined.

This ended up being crucial because the first week or so my family member was unsure about the medication but it seemed after the time period that had been discussed my family member was feeling less up and downs and more regulated at that time.

Doctors are the experts of course, but in my opinion they need specific feedback from the patient, and in some cases family members to help them figure out the best medication and dose.

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