L14. Medicine Interactions

Question 1

Define drug interaction

Answer 1

When the effects of one drug are changed when administered with another drug, food, or other substance.
Could be increased, decreased or new.

Question 2

Define Drug-herb interaction

Answer 2

Interaction of drug with a natural health product

Question 3

What does synergism, potentiation, additivity and antagonism mean for drug interaction

Answer 3

Syn: effect of drugs given together > effects drugs given alone
Potentiation: effect of one or more drugs when given with an interacting drug > when acting alone.
Additivity: effects of drugs given in combo = is expected from the effects alone
Antagonism: effects of drugs in combo < when given alone

Question 4

What are the 3 ways that drug interactions are classified based on their underlying mechanism

Answer 4

1. Pharmacokinetic: one drug affecting the other’s absorption, distribution, metabolism or elimination
2. Pharmacodynamic:
   the effect of two or more drugs on the same receptor or physiological system without change in drug conc
3. Pharmaceutical: drug interaction in syringes or infusion fluids due to direct chemical interaction outside the body.

Question 5

What are the effects of a drug altering the rate or degree of absorption of another drug in the GI tract- more, less,
Give one example

Answer 5

• Increased absorption = exaggerated, prolonged activity +/- toxicity
• Decreased absorption = loss of therapeutic function

eg. Metal containing antacids or supplements decrease the absorption of antibiotics by binding to the antibiotic to form a non soluble, metal chelate.

Question 6

How does one drug induce the metabolism of another

Give example

Answer 6

One drug can induce enzymatic activity to metabolise the other drug which occurs over several days- weeks, frequently uses Cytochrome P450 enzymes and is regulated at the level of transcription.
The effect depends on activity level of metabolites - loss therapeutic or gain/toxicity.

Eg. Anticonvulsant Carbamazapine or phenytoin will increase metabolism of warfarin and oral contraceptive - reduced anticoagulant or contraceptive effect

Question 7

How does one drug inhibit the metabolism of another

Give example

Answer 7

Inhibition of drug metabolising enzyme eg. Cytochrome P450 enzymes or aldehyde dehydrogenase, that have a rapid onset of effect within hours.
It can cause exaggerated or prolonged responses or increased toxicity.

Eg. Fluconazole inhibits warfarin metabolism- increased effect

Question 8

How does one drug alter the elimination of another and what is the general effect
Give example

Answer 8

Drug competes for renal tubular secretion for renal or biliary elimination with the other drug. Inhibits their transport molecule. This may slow drug clearance causing exaggerated +/ prolonged activity +/ toxicity

eg. Erythromycin inhibits renal P-glycoprotein which eliminates digoxin leading to digoxin toxicity

Question 9

What are the two different effects of pharmacodynamic drug interactions.
Give an example

Answer 9

Potentiation or
Antagonism.

eg. potentiation of anti-coagulation: warfarin + aspirin = higher risk of bleeding

Question 10

How can we reduce harm from drug interactions?

Answer 10

-Knowing the common perpetrator drugs involved - azole antifungals, macolides (thromycins), anti-convulsants

• Knowing the drugs with low therapeutic index: susceptibles:
  anti-coagulants, anti-arrhythmics, antiepileptics
• Know the drugs you frequently prescribe
• Know drugs of low risk for common problems
• Know how to recognise high risk patients : extremes of age, high number of medicine, organ impairment.

Question 11

Where can you find information about drug interactions

Answer 11

NZ formulary, Med safe data sheets.

Also Hospital medicine information service, prescribing and dispensing software and online tables.