PHC5005 Chap.1 Drug Absorption, Routes and Bioavailability
Drug Absorption, Routes and Bioavailability
Where pharmacokinetics begins
Pharmacokinetics covers the body's handling of a drug, and the course opens it with the first of four processes: absorption, distribution, metabolism and excretion. Absorption is the transfer of a drug from the place it is given into the bloodstream.
Every route except intravenous injection has to achieve that transfer across at least one membrane, and the speed and completeness of the crossing decide when an effect starts and how large it can become.
Why a tablet takes an hour
An oral dose must leave the stomach, dissolve, and cross the gut wall.
The patient controls part of that: gastric emptying, gut contents, the health of the intestinal lining and the surface available for absorption. Diabetic gastroparesis slows the stomach and delays oral hypoglycaemics; an older patient may absorb more slowly because gut motility falls. Food delays tetracycline and morphine, while a fatty meal improves albendazole.
The drug controls the rest: small, lipid-soluble, unionised molecules cross by passive diffusion, while large or water-soluble molecules need carriers or receptor-mediated uptake.
Weak acids and weak bases change their ionised fraction as the pH changes along the gut, yet most absorption still happens in the small intestine because of its surface area.
Routes as trade-offs
The course groups routes into enteral, parenteral and topical or local families and compares them on speed, completeness, convenience and risk.
Oral dosing is easy, cheap and self-administered but slow and exposed to first-pass loss. Sublingual and rectal routes avoid much of that loss. Intravenous dosing is immediate and complete, and it also carries the greatest risk of systemic adverse effects and needs a trained person. Transdermal patches trade speed for steady, prolonged delivery.
The right route is the one that fits the patient in front of you, which is why the lecture frames it through short clinical scenarios rather than a ranking.
Bioavailability and bioequivalence
Bioavailability is the fraction of the dose that reaches the systemic circulation.
It is reduced most by first-pass metabolism in the gut wall and liver, which is why oral nitroglycerin fails and the sublingual tablet works. Low bioavailability can mean a drug never reaches therapeutic levels; high bioavailability can mean efficacy and sometimes toxicity.
Two formulations of the same drug are bioequivalent when the speed and the completeness with which the active drug reaches the circulation do not differ significantly, and a generic product must prove this before it can replace the branded one.
Why it matters beyond the ward
The lecture closes with two lists of consequences.
Clinically, absorption knowledge guides route and formulation, informs dose changes in special populations and helps predict interactions. In drug development it shapes formulation design, feeds the ADME profile in preclinical models and supports first-in-human studies. A newer strand, the influence of gut microbiota on oral bioavailability, is introduced as a direction personalised medicine may take.
What this chapter covers
- 01
Absorption as the passage of a drug from wherever it is given into the bloodstream
- 02
Patient factors: absorptive surface, gastric emptying, gut contents, disease and age
- 03
Drug factors: molecular size, lipid solubility, ionisation and formulation
- 04
Enteral, parenteral and topical routes compared on speed, completeness and risk
- 05
First-pass metabolism and the bioavailability ranges of each route
- 06
Bioequivalence between branded and generic products
- 07
Gut microbiota as an emerging influence on oral bioavailability
Worked example · free
Choosing a route when the gut is unavailable
- 1Rule out the oral route and say why.
- 2Name the route that still uses the gut but not the stomach.
- 2State the trade-off you accept by choosing it.
Key terms
- Absorption
- The passage of a drug from wherever it was given into the bloodstream, the central compartment.
- Bioavailability
- The fraction of an administered dose that reaches the systemic circulation, 100 per cent by definition after intravenous injection.
- First-Pass Metabolism
- Pre-systemic loss of an oral drug in the gut wall and liver before it reaches the general circulation.
- Bioequivalence
- Agreement between two preparations in how fast and how completely the active drug reaches the circulation, tested under suitable conditions.
- Enteric Coating
- A gastric-resistant tablet coat that lets a drug such as omeprazole pass through stomach acid intact.
- Generic Drug
- A product containing the same active ingredient as the branded original, sold under its non-proprietary name after the patent expires.
- Transdermal Patch
- A skin-applied system giving slow, prolonged and steady absorption without first-pass loss, used for nicotine and fentanyl.
- Depot Injection
- An injected preparation that releases drug slowly over a long period, such as medroxyprogesterone.
Drug Absorption, Routes and Bioavailability FAQ
Why is the small intestine the main site of absorption for most oral drugs?
Its lining carries microvilli and a rich splanchnic blood supply, so the absorptive surface and perfusion far exceed the stomach's. Even a weak acid that is mostly unionised in gastric acid is absorbed in greater total amount further down, because area and blood flow outweigh the ionisation advantage.
Which routes avoid first-pass metabolism?
Intravenous, intramuscular and subcutaneous injection bypass it, as do transdermal, inhaled and other topical routes. Sublingual dosing bypasses the gastric environment and so loses less of the dose before it reaches the circulation. Rectal dosing reduces it compared with swallowing, though absorption there is erratic.
How does food change the absorption of a drug?
It depends on the drug. A full stomach delays tetracycline and morphine, a fatty meal improves albendazole and griseofulvin, and an empty stomach lets ibuprofen act faster. The general principle is that gut contents alter both the rate of emptying and the chemistry the dose meets.
What does a bioavailability of 25 per cent mean for dosing?
Only a quarter of an oral dose reaches the circulation, so matching an intravenous dose needs roughly four times as much by mouth. The course cites propranolol at about that level and names a higher oral dose or the intravenous route as the alternatives.
Can two tablets with identical active ingredients differ clinically?
Yes. Chemically equivalent products from two manufacturers may not produce the same blood levels. That is why a generic must demonstrate bioequivalence with the reference product, meaning its active drug reaches the circulation as fast and as completely as the reference, within limits a suitable test cannot distinguish.
Why must an enteric-coated tablet not be crushed?
The coating exists to carry the drug through stomach acid without being destroyed. Crushing or chewing it removes that protection, so a drug such as omeprazole meets the acid it was designed to avoid and less active drug survives to be absorbed.
Exam move
Build one table from memory with the routes down the side and four columns across: onset, stated bioavailability range, first-pass exposure and one situation where it is the right call. Then test it on the lecture's scenarios, a headache at home, a sprained ankle, chronic pain and a feverish baby, before checking against your notes.
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