Medication elimination from the body

How long will medication stay in my body after I stop taking it?

The length of time medication remains in the body depends on the type of medication. The speed at which it is removed is measured in “half-lives.” A half-life is the amount of time it takes for the level of medication in the blood to fall by half. Medications have a wide range of half-lives. For example, some are as low as 1 1/2 hours and others can take over 100 hours. Mothers who take medication that is not safe during breastfeeding can generally resume breastfeeding after five half-lives. At that point, only 3% of the starting dose will be present. If the drug has a half-life of three hours, the mother could typically resume breastfeeding after five half-lives, or 15 hours. More toxic chemotherapy drugs require a minimum of seven half-lives.

A) Measuring removal of medication

1) Half-life of a medication

The body will remove medication in various ways, including through the urine or by being processed in the liver. This happens at various rates, depending on the medication.

The speed at which it is removed from the blood is measured in units known as half-lives. Half-life is shortened to . The “t” refers to time and a t½ is the amount of time it takes for the level of medication in the blood to fall by half.

Medications have a wide range of half-lives. For example, salbutamol, used by people with asthma, has a half-life of 1.6 hours and diazepam, used to treat seizures and anxiety, has a half-life of 100 hours.

2) How half-lives work

Consider a medication with a half-life of three hours. After three hours, the blood level will be one-half of the original level. After another three hours, the level will have dropped again by one-half of that half, or one-quarter of the original amount. That leaves one remaining quarter, or 25% of the original amount.

If this is confusing, think of cutting an apple into two equal parts and keeping only half of what you cut. The first cut leaves you with half an apple, the second cut leaves you with one-quarter, the third cut leaves you with one-eighth, and so on. The following table illustrates this.

Table: Relationship Between a Medication With a Half-Life of Three Hours and the Amount of Drug Present in the Blood

 B) Using half-lives to predict when breastfeeding will be safe

Mothers who take medication that is not safe during breastfeeding can generally resume breastfeeding once five half-lives have passed. Only 3% of the starting dose will be present at that time.  

In the above example, the half-life is three hours, so the mother could generally safely breastfeed 15 hours (5 half lives x 3 hours) after the last dose of medication.

For more toxic drugs, such as chemotherapy drugs, seven half-lives are considered a minimum.