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Drug administration and dosing regimens

Transcript

Content Reviewers:

Yifan Xiao, MD

Every medication can be administered through various means, known as routes of administration; and various schedules, meaning at a particular frequency and amount, or dose, which is known as the dosing regimen.

Alright, so routes of administration are broken down into three main categories: enteral, parenteral, and topical.

In enteral administration, which is the most common form, the medication is administered through the gastrointestinal tract.

This could mean that it’s swallowed by the mouth, also known as peroral administration; placed under the tongue, also known as sublingual administration; between the gums and the inner lining of the cheek, also known as buccal administration; or finally, into the rectum, also known as rectal administration.

On the other hand, parenteral administration includes any route that bypasses the gastrointestinal tract, to pump the medication directly into the circulation, such as through an injection into a vein - intravenously, or IV for short; under the skin - subcutaneously, or SC for short; or into muscle - intramuscularly, or IM for short.

Finally, there’s topical administration, where the medication is applied directly upon a particular area of the skin or mucous membrane to achieve a local effect. For example, antifungal creams to treat athlete's foot.

Now, choosing the route of administration depends on many factors.

First of all, these include the chemical properties of the medication itself, such as its stability, and its ability to cross certain barriers of absorption.

For example, a perorally administered medication needs to be able to resist tough, acidic conditions within the stomach, and then readily pass through the walls of the intestines, into the blood.

In addition to this, blood coming from the gastrointestinal tract is first directed to the liver.

And that’s where many medications get broken down or metabolized before gaining their entry ticket to the systemic circulation.

This means the medications are metabolized before they even get a chance to reach their target tissues.

This is known as first-pass metabolism, or the first-pass effect.

So, medications that get extensively metabolized into inactive forms through that first-pass shouldn’t be administered perorally.

In contrast, IV administration allows medications that are less stable, or less capable of being absorbed through the gastrointestinal tract, to be directly administered into the bloodstream, which is also beneficial for medications that have a major first-pass effect.

Next, another factor that should be taken into account is the urgency of the situation.

So for example, peroral medications take time to get absorbed in the GI tract, so they produce an effect much slower than intravenous ones.

And for this reason, intravenous administration is usually preferred in emergency settings and during surgical procedures.

On the flip side, oral preparations are much easier to take at home and don’t need any special equipment.

Alright, now each medication is given at a specific schedule or dosing regimen, which determines the frequency of administration and the amount administered, known as dose.

The regimen is important since it affects the onset of action, or the time it takes for a medication to start working and produce an effect, as well as the duration of action, which is the total length of time during which a medication produces an effect.

So, there are three main types of dosing regimens.

First, there’s single dosing, meaning that only one dose of a medication is administered.

Let’s plot this into a graph with the drug concentration in plasma on the y-axis, and the time on the x-axis.

So, the plasma concentration of the medication rises steadily as it gets absorbed into the circulation, until it reaches a peak.

Then the concentration drops again, as the medication is distributed into tissues, get metabolized, or is eliminated.

Sources
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