MDG5204 Chap.1 Drug Absorption, Routes and Bioavailability
Drug Absorption, Routes and Bioavailability
Where the course begins
The foundations theme opens with the four processes that decide what happens to a dose between the moment it is given and the moment it is gone. Absorption is the first of them, and the lecture defines it as the transfer of a drug from wherever it was given into the central compartment, which is the blood.
A drug that is never absorbed cannot be distributed, metabolised or excreted, and cannot act, which is why so many clinical puzzles about a drug that seems not to work end up here.
Patient factors and drug factors
The lecture sorts everything that changes absorption into two lists.
On the patient side are the size and blood supply of the absorbing surface, how quickly the stomach empties and the gut moves its contents, what else is in the gut, disease of the gut wall, and demographic factors such as age and sex. On the drug side are lipid solubility, molecular size, ionisation at the local pH, solubility, formulation and the route itself.
The summary line the course wants remembered is short: small, unionised, lipid-soluble molecules cross membranes best, although a drug must also dissolve before it can cross anything.
Routes and the problem of first pass
Routes fall into enteral, parenteral and topical or local families, and each is taught through a table of absorption pattern, advantages, disadvantages and examples.
The idea that links the table together is first-pass metabolism: drug absorbed from the gut goes through the gut wall and liver before reaching the systemic circulation, and some of it is metabolised on the way. Intravenous, intramuscular and subcutaneous injection bypass that loss; sublingual and rectal routes reduce it.
Drugs such as nitroglycerin, propranolol, salbutamol, lidocaine and the statins are the lecture's examples of drugs where first pass changes the route or the dose.
Bioavailability and bioequivalence
Bioavailability is the fraction of an administered dose that reaches the systemic circulation, and it is measured by comparing the area under the plasma concentration curve after an oral dose with that after the same dose given intravenously.
The ranges the course gives run from 100% for intravenous injection down to as little as 5% for oral and inhaled drugs. Bioequivalence extends the idea to products: two formulations of the same drug are bioequivalent when their bioavailability shows no significant difference in speed or amount, which is what a generic must show before it can replace a branded product.
What this chapter covers
- 01
Absorption defined as transfer from the point of dosing into the blood
- 02
Patient factors: absorptive surface, gastric emptying, gut contents, disease and demographics
- 03
Drug factors: lipid solubility, size, ionisation, solubility and formulation
- 04
Enteral, parenteral and topical routes compared on speed and completeness
- 05
First-pass metabolism and the drugs whose route it changes
- 06
Bioavailability from the ratio of oral to intravenous AUC, and bioequivalence of generics
Worked example · free
Choosing between a higher oral dose and a new route
- 2Relate the oral dose to the IV dose through F.
- 2Compute the equivalent oral dose.
- 2Name the alternative the course would reach for.
Key terms
- Absorption
- The transfer of a drug from wherever it was given into the blood.
- Enteral Route
- Administration through the gastrointestinal tract: oral, sublingual or rectal.
- Parenteral Route
- Administration by injection, such as intravenous, intramuscular or subcutaneous, bypassing first-pass metabolism.
- Bioequivalence
- Equality of the rate and extent of bioavailability between two formulations of the same active drug.
- Generic Drug
- A product with the same active ingredient as the branded original, sold under a non-proprietary name after the patent expires.
Drug Absorption, Routes and Bioavailability FAQ
Why does a drug given by mouth take longer to act than the same drug given intravenously?
An intravenous dose is already in the circulation, while an oral dose must dissolve, cross the gut wall and pass the liver before it arrives. Gastric emptying, food and gut motility all slow that journey, which is why an oral antihypertensive can take about an hour to show any effect.
Which routes of administration avoid first-pass metabolism?
Intravenous, intramuscular and subcutaneous injection avoid it completely because drug enters the systemic circulation without passing the liver first. Sublingual and rectal routes reduce it, and transdermal, inhaled and other topical routes also avoid the portal circulation.
Can a generic medicine work differently from the branded one?
Two products can contain the same active ingredient and still differ in how fast and how completely it reaches the blood, which is why generics must demonstrate bioequivalence. Once that is shown, differences in shape, colour and price do not change the expected effect.
Exam move
Learn the route table as a set of trade-offs rather than a list: for each route, say how fast, how complete, and why a clinician would choose it. Then practise converting between oral and intravenous doses using bioavailability until the direction of the division is automatic.
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