20 Must-Know Routes of Administration!

What are routes of drug administration?

In pharmacology, “routes of administration” are the pathways by which drug is administered to a patient.

For example, some drugs are administered orally, whereas other drugs are administered via the rectal or intravenous route – directly into a vein. Generally, we classify routes of administration into three categories:

• Enteral – through the gastrointestinal tract
• Parenteral – through any route that does not involve the GI tract
• Topical – through the skin

Why, then, is one route of administration chosen over another? To answer this question, we need to consider the following five factors:

• Physical and chemical properties of the drug.
• Site of action – is local or systemic effect required?
• Rate of drug absorption from different body sites.
• Impact of gastrointestinal enzymes / liver metabolism on the drug.
• State of the patient – are they conscious or unable to take oral drugs, for example?

Some drugs cannot be administered via the oral route – such as insulin – because gastric acid destroys the drug. For this reason, insulin is administered via a parenteral route, such as the subcutaneous route. Some drugs work best when administered direct to the site of action, such as the skin, rather than through systemic means, such as the IV route. The rate and extent of drug absorption is different for different sites of the body. Some drugs are absorbed more from one site than another. In other cases, absorption may be intentionally slowed to ensure a consistent rate of drug is fed into the body. Furthermore, many drugs are metabolised by digestive enzymes, gut wall enzymes, and liver enzymes.

The rate and extent of drug absorption from different sites is dictated, in part, by the physical and chemical properties of the drug. Some drugs have greater solubility in fat, whereas others do not. These are the kinds of factors that influence which route of administration is chosen.

First-Pass Effect and IV Administration

Many drugs are metabolized, even destroyed by the gastrointestinal tract – more specifically, by the gut wall and liver metabolism.

This is referred to as the first-pass effect, also known as ‘pre-systemic metabolism’ – where digestive and liver (hepatic) enzymes transform and metabolise the drug into another product, often a product that loses its original clinical utility. For this reason, many drugs are administered via parenteral routes – such as the intravenous route – to avoid these digestive enzymes, delivering the medicine direct into the bloodstream.

By giving drug direct into veins, it ensures a more rapid effect. Taking medicines via the oral route takes time – the drug must be digested, metabolized and absorbed. This isn’t the case with the intravenous route, which injects the medicine into a vein for direct effect. This may be necessary in cases where patients cannot swallow or in patients who are unconscious or in cases where a rapid systemic effect is needed – such as in an emergency situation. Similarly in patients suffering from emesis (vomiting), it may be prudent to offer a route of drug administration that bypasses the digestive tract, otherwise the medicine may also be vomited up. In these cases, parenteral routes of administration becomes essential.

In the case of intravenous administration, there are three means of delivery:

• Bolus IV – drug administered over a short period (known as IV push; IVP).
• Continuous IV infusion – drug administered to the patient over hours or days by continuous drip or infusion.
• IV piggyback (IVP) – small volume of IV medicine given in addition to an IV infusion; “piggybacking” through the primary IV line.

Routes of Administration

Below we have covered all major routes of drug administration, all of which are examinable on the PTCB test.