The term “half-life” is often heard when talking about medicine. It may also be discussed in terms of withdrawing from a drug.

This article looks at what a medicine’s half-life is and how it is important in the field of mental health. It is very useful information to have.

File Photo: A pharmacy

What does a medicine’s half-life mean?

The half-life of a medicine is the length of time it takes for the level of a drug’s active ingredient/substance in your body to reduce by half [1].

Many people explain it by saying calling it an “elimination half-life” – as if to say that it is the length of time it takes to eliminate half of the drug from the body.

Essentially, this means that if a person takes a 100mg dose of anything – whether it is Paracetamol, Ibuprofen or any prescribed medicine, the length of time it takes for the active ingredient to be at 50mg in the body.

The half-life of a drug can vary hugely. It can vary from a few hours to a few days, or even a few weeks in some cases.

What affects the half-life of a medicine

The main cause of differences in half-lives of medicines is the drug itself. Each medicine – depending on the exact make-up of the ingredients – will have a different half-life.

Elimination of the medicine can range in length of time depending on the person. Factors that have an impact on half-life include age and weight.

The health of a person’s kidney and liver is also a consideration – as this is where many drugs are excreted. Moreover, if a person takes multiple medicines, this can also affect the half-life.

Interestingly, regardless of the dose that a patient is on or the length of time they have been taking the drug – its half-life will always be the same.

The impact of a medicine’s half-life on withdrawal?

The half-life of a medicine is an important consideration when it comes to withdrawing from a medicine. The aim is to always withdraw from a medicine – rather than staying on it for life.

If a medicine has a short half-life, it will generally be more difficult to withdraw from it. Conversely, if a medicine has a long half-life, it is generally easier to withdraw from it.

If a patient is withdrawing from a medication with a short half-life, they can sometimes be given a medicine that has similar properties, albeit with a longer half-life.

For example, the antidepressant Fluoxetine (Prozac) is renowned for having a long half-life. Therefore, if a person is struggling to withdraw from another antidepressant that has a short half-life like Venlafaxine (Effexor), they may be given Fluoxetine instead to aid the withdrawal process.

Using the above medicines as an example, the half-life of Fluoxetine is up to 6 days. However, for Venlafaxine, it is generally 8 hours maximum. Making it difficult to withdraw from.

Many recreational drugs have a short half-life and therefore to keep the “high”, a person needs to take it more often. This increases the process of dependence.


If you have any queries about your medicine, it is very important to have a discussion with your GP or mental health professional. They will be able to answer specific questions.

The topic of a medicine’s half-life is important. It is something that should be considered when a patient is being prescribed a medicine and when they come to withdraw from it.

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[1] Hallare, J., & Gerriets, J. (2022). Half Life. Available: Last accessed: 8th May 2023.