Drug interactions

Question 1

What is a drug interaction?

Answer 1

A change in one drugs effect when administered with another drug, food, or other substance.

Question 2

What are pharmacokinetic interactions?

Answer 2

Where the interacting drug alters the plasmaa concentration of one or both drugs.
These interactions can occur at the level of absorption, distribution, metabolism or elimination.

Eg at the level of absorption some drugs such as calcium binds to other drugs and prevents their absorption. Other drugs may alter the pH affecting solubility etc.

Question 3

Describe the time course of enzyme inhibitors compared to enzyme inducers

Answer 3

Enzyme inhibitors act rapidly to increase the conc of substrates in the blood (time is determined by half life of the drug and time of the inhibitor drug to reach steady state as well as time of the substrate to reach a new steady state conc).

Question 4

CYP3A4 is one of the most common CYP enzymes (metabolizes more than 50% of medicines). What are some substrates, inducers and inhibitors of this enzyme

Answer 4

Substrates = statin, Ca2+ channel blockers, HIV antivirals
Inhibitors = Grapefruit juice
Inducer = St Johns Wort

Question 5

What is a dangerous drug interaction St Johns wort can cause?

Answer 5

It has cause organ rejection when added to cyclosporin therapy as it enduces the enzyme that breaks down this immunosuppressant.

Question 6

What are pharmacodynamic interactions?

Answer 6

Where a drug affects the effect of another drug not the concentration.

Question 7

What is serotonin syndrome?

Answer 7

If you take two drugs that increase serotonin levels it can result in too much serotonin which can lead to hopitilization due to fever, sweating, agitation etc.

Question 8

How can doctors predict the interaction of medications?

Answer 8

Medsafe website and New Zealand formulary website