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How Does Drug Half-Life Work?

A drug’s half-life refers to the amount of time it takes for half of a substance to be flushed out of the body by natural processes.

Most of the time, this process involves processing by the liver and kidneys and then elimination via excretion. Each substance has a different half-life, with some being only minutes and others being days.

Certain medications have larger half-life ranges than others as well. No drug has an exact half-life period due to the fact that human factors influence the process of elimination of any substance. Morphine, for example, has a half-life of only 2-3 hours, while Klonopin has a half-life of 18-50 hours.

What Half-Life Can Tell Us

A drug’s elimination half-life can predict how addictive a drug is and how severe withdrawal from the drug is likely to be. Though it depends on the drug, it’s often true that substances with short half-lives produce more intense withdrawal symptoms than those with longer elimination periods. Generally, if the drug leaves the body faster, an individual will need to take more of the drug more often in order to maintain a high, get relief from pain, or treat any kind of medical condition.

If taken for pain or pleasure, the more the drug is taken, the more the reward response is reinforced in the brain. Part of addiction hinges on this response as the brain creates an association between the action of taking the drug and the “reward,” whether that’s pain relief or a pleasant high. This association is what can produce addiction to substances that are not considered physically addictive and even to behaviors and activities.

Withdrawal symptoms are also often a key part of addiction, at least with physically addictive substances. These symptoms can be so severe that they serve as a deterrent to even attempting to quit. Common withdrawal symptoms include:

Having to face withdrawal is likely why, according to the Substance Abuse and Mental Health Services Administration’s National Survey on Drug Use and Health, only 11.2 percent of people with an addiction disorder received treatment for the condition in 2009.

Factors Affecting Half-Life

There are several factors that influence how long it will actually take for a drug to completely leave one’s system.

These can include:

  • Body weight
  • Body fat percentage
  • Age
  • Gender
  • Genetics
  • Liver and kidney health
  • General nutrition
  • Metabolism speed
  • Amount of the drug taken before stopping

The amount of body fat a person has can significantly affect a half-life period due to the fact that, especially during binges, excess intoxicants in the body will be stored in fat cells. It takes longer for the drugs stored this way to be flushed out of the system. This also makes gender a factor due to the fact that women naturally have more body fat than men. Older individuals also tend to metabolize drugs at a slower pace than younger people, though each person’s natural metabolism rate varies.

People who have liver and kidney issues may also take longer to eliminate drugs from their system due to the simple fact that these are the organs responsible for processing and eliminating toxins from the body. General nutrition influences almost all other factors; healthy people tend to have less body fat and process things efficiently.