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In Medicine, what is a Bolus?

By Alex Terris
Updated Feb 16, 2024
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In medicine, the term bolus is used to refer to a drug that is given to a patient in order to increase concentration in the blood. Usually, a bolus is a small ball of medicine. There are a number of different ways that a bolus can be delivered such as intravenously and intramuscularly. A common example of a bolus is the administration of insulin to people who suffer from diabetes. A bolus is also the name given to a large pill that is given to cattle and other animals in order to prevent dehydration.

Where the bolus is administered in the body is an important factor in order to ensure that the medication reaches the correct concentration in the appropriate time. The purpose is to raise the concentration to an effective level — the level at which 50% of people see the desired response from a particular drug. For this reason the placement of the medical pill depends on the current concentration levels in the body.

The method of delivery also depends on the desired effect. For example, the medication may be given intramuscularly in order to provide a slow release. This is especially useful during the injection of vaccines that need to develop the body’s immune system over time. Subcutaneous injections, on the other hand, are commonly used by drug addicts.

An intravenous injection is the most effective at increasing the concentration quickly. For this reason it is most commonly used when it’s important to raise concentrations in a timely manner such as after a treatment. Intravenous injections of medicine are also commonly used after medicine has been removed from the body of a patient.

Many health care workers regularly refer to a bolus as a dose of insulin that is quick to act. A diabetes dosage must be taken during a meal as it is fast acting. On the other hand, there are methods of insulin administration that are much slower to act and hence are used for other purposes. For example, a basal rate provides a much slower dose of insulin.

The term is also used for some drugs given to animals by a veterinary professional. These are used for a number of different purposes including to counteract kidney problems and to act as a slow release dehydration tablet. Due to the fact that there are several minutes between administration and absorption, during this time the medicine is a bolus.

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Discussion Comments
By indigomoth — On Nov 25, 2011

@Mor - We had the same problem one year with some of our calves. I was quite young and can't remember exactly what was wrong with them, but they were sick and became quite dehydrated.

I remember when the vet came to look at them, he injected some saline and some medicine into their haunches.

He told me to rub firmly at the place where he had injected, in order to spread the medicine. It felt like there was a bubble under the skin and it took what seemed like a while to go away, but was probably only a few minutes.

I was amazed at how quickly the calves seemed to feel better.

Afterwards I wanted to be a vet for a long time, just because he seemed like such a miracle maker.

By Mor — On Nov 25, 2011

I think a bolus can refer to quite a few things, but is often a lump of medicine under the skin.

I had a kitten a few years ago who got sick really fast, and unfortunately it was summer, so she was affected by dehydration. She become really limp and dull eyed and I decided I needed to get her to the vet right away.

Luckily the vet was willing to see me quickly, because he said if we'd waited any longer she could have died. He produced this enormous needle, which terrified me (it was almost the size of the cat!) and injected what he called a "bolus" under her skin, which was basically a lump of saline solution to hydrate her again.

That lump stayed there for a while, but gradually she started looking brighter and more bushy tailed again and then she was fine. He said it often happens that new kittens get sick and go downhill fast, so it's a good idea to keep a close eye on them.

By bythewell — On Nov 24, 2011

I've also heard it called bolus when referring to what some people do when injecting heroin.

They inject the drug under the skin so it doesn't enter the blood stream all at once, but soaks in slowly.

Even thinking about it makes me uncomfortable, to be honest, but I've done some research on it for an article I'm trying to write about drug culture.

Apparently by doing this kind of injection, they can avoid the rush but still get the edge taken of the cravings.

I have no idea if it can be used as a way of slowly weaning yourself off the drug, or if it ends up making it worse since you get used to a more sustained high. I guess people do it in order to function better in society without the rush.

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