Wattle Pharmacy Prep

Absorption and bioavailability
Practice questions with explanations

Absorption and bioavailability belongs to Pharmacokinetics and pharmacodynamics, which makes up about 10% of the Overseas Pharmacist Readiness Assessment (OPRA®). Answer each question yourself before opening the explanation.

1. Which factor determines whether a drug can be given rectally?

  1. Any oral formulation can be given rectally
  2. The route is never useful
  3. Adequate absorption across rectal mucosa, tolerability, and an appropriate formulation, with partial avoidance of first-pass metabolism
  4. Rectal absorption is always complete
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C. Adequate absorption across rectal mucosa, tolerability, and an appropriate formulation, with partial avoidance of first-pass metabolism

The route is valuable when a patient cannot swallow or is vomiting, and it is used in paediatrics and palliative care for antiemetics, analgesics and antiepileptics. Absorption is variable, cultural acceptability differs, and the formulation must be designed for the route rather than improvised from an oral product.

2. How do antacids and proton pump inhibitors affect the absorption of some drugs?

  1. They increase absorption of all drugs
  2. They have no effect on other medicines
  3. By raising gastric pH they reduce dissolution of drugs needing an acid environment, such as some azoles and iron salts
  4. They act only on the drugs' metabolism
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C. By raising gastric pH they reduce dissolution of drugs needing an acid environment, such as some azoles and iron salts

Drugs that are weak bases dissolving best in acid — ketoconazole, itraconazole capsules, atazanavir, ferrous salts — are absorbed less when the stomach is alkalinised, sometimes enough to cause treatment failure. Antacids add a chelation problem for tetracyclines, quinolones and bisphosphonates, which is solved by separating doses; the acid suppression problem is not solved by separation.

3. How does an intramuscular injection differ from a subcutaneous one in absorption?

  1. Muscle has richer blood flow, so absorption is generally faster than from subcutaneous fat
  2. Subcutaneous absorption is always faster
  3. Neither route allows systemic absorption
  4. Both routes give identical rates for every drug
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A. Muscle has richer blood flow, so absorption is generally faster than from subcutaneous fat

Greater perfusion in muscle speeds removal of drug from the injection site, so intramuscular administration usually acts faster, which is why adrenaline in anaphylaxis is given intramuscularly into the anterolateral thigh. Absorption from either site falls in shock, when peripheral perfusion collapses. Formulation can override the anatomy, as with oily depot injections designed for slow release from muscle.

These are 3 of the 31 questions on absorption and bioavailability in the app.

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