What is a Bolus? - Definition, Administration & Biology

Instructor: Artem Cheprasov
This lesson is going to define the word bolus from more than one perspective. Then you'll learn how boluses may be administered and why they are given in general.

What is a Bolus?

At one point in your life, you probably took a bite of a large piece of bread. Then, you chewed it. As you chewed it, that chunk of bread got mixed with saliva and formed into this relatively large mass of food inside of your mouth, which you then proceeded to swallow.

Do you know the proper name for that chunk of food that you just swallowed? The actual mass itself? Yes, it is bread, but it's also called something else now, something known as a bolus of food.

In this lesson, we're going to define bolus, find out how boluses are administered in medicine, and the biological implications behind these methods of administration.


So, as per the introduction, bolus, can be very broadly defined as a mass of some sort of substance that is about to be passed into, or is already inside of, some sort of tube-like structure of the body. What tube-like structure? This namely refers to the gastrointestinal tract and the veins of the body.

In medicine, a bolus is often a pharmaceutical substance intended to have a therapeutic effect. Oftentimes, the term bolus also implies that the mass or quantity of the preparation given is relatively large in size and/or quantity with respect to the relatively short time-frame within which it is administered.


Examples of boluses include:

  • A mass of solid or semisolid food that has just been chewed and mixed with saliva, ready to be swallowed.
  • That same mass of food above passing through the digestive tract, but prior to being fully digested.
  • Large oral preparations of medications or diagnostic preparations in solid (tablet), semisolid, or liquid form to be given all at once or in a very short time-frame.
  • Large (by quantity and/or dose/concentration) amounts of medications or diagnostic preparations injected all at once into a person's veins (intravenously).

More loosely, boluses can be taken to mean any form of therapeutic or diagnostic preparation given in a large amount or in a high dose via any route of administration within a very short period of time. For example, the administration of a relatively large amount of injectable medication subcutaneously can be considered a bolus as well.


So why do we bother giving boluses in medicine? Well, biologically speaking, there may be more than one reason.

First, there could be a practical aspect to it. Imagine you are a veterinarian working with large animals, like cattle. Getting these animals to comply with taking an oral medication is not very easy. You have to round the animals up, isolate them into stalls, and get them to stop moving their head within the stall. This is no easy feat with massive animals like this. Then, you have to administer the medication using a gigantic pill gun. The animals don't like this and will resist. This is labor and time intensive work.

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