YW - Drug Interactions

Question 1

What are drug interactions? (3)

Answer 1

Can occur whenever 2 or more drugs are given

• It is the modification of the effect of one drug (the object drug) by the prior or concomitant administration of another.

The pharmacologic or clinical response to the administration of a drug combination different from that anticipated from the known effects of the 2 or more agents when given alone.

Question 2

What are 2 types of drug interactions?

Answer 2

May be harmful: toxicity, reduced efficacy;

May be beneficial: synergistic combinations, pharmacokinetic boosting, increased convenience, reduced toxicity, cost reduction

Question 3

What are 3 reasons for multiple drug therapy

Answer 3

1. Combination is better than a single agent
2. Patients have more than one disease
3. Drug B may reduce/block the side effects of drug A

Question 4

What are 2 types of risk factors for drug interactions?

Answer 4

High Risk Patients:

• Elderly
• young
• very sick
• patients with multiple disease (especially with liver and/or renal impairment) that require multiple drug therapy

High Risk Drugs:

• Narrow therapeutic index drugs e.g. (digoxin, warfarin, theophylline)
• Recognised enzyme inhibitors or inducers

Question 5

Which age group is most at risk of ADRs?

Answer 5

The elderly are most at risk of adverse drug reactions- ADRs

• DUE TO POLYPHARMACY

Question 6

Additivity, Antagonism, Potentiation and Synergism in drug interactions

Answer 6

additivity: a combination of two or more chemicals is the sum of the expected individual responses

antagonism: exposure to one chemical results in a reduction in the effect of the other chemical

potentiation: exposure to one chemical results in the other chemical producing an effect greater than if given alone

synergism: exposure to one chemical causes a dramatic increase in the effect of another chemical

Question 7

What are 3 types of drug interactions?

Answer 7

1. Chemical interactions – drugs combine chemically
2. Pharmacodynamic interactions – drugs interact at the receptor
3. Pharmacokinetic interactions – altering the concentration of the drug that reaches the receptor

Question 8

Describe chemical interactions (outside the body) in drug interactions

Answer 8

Incompatibilities: reaction of iv drugs resulting in solutions after mixing that are no longer safe for the patient due to altering stability (change the PH) or structure,Penicillin and aminoglycoside should never be placed in the same infusion fluid because of formation of inactive complex leading to:

• Loss of drug activity
• Formation of precipitates
• Development of toxic product

e.g. Penicillin and aminoglycoside should never be placed in the same infusion fluid because of formation of inactive complex

e.g. Heparin is highly negatively charged and will combine with basic drugs. IV lines are therefore often flushed with saline before a drug is given

Question 9

Drugs competing for the same receptor site may involve what and what is cross tolerance?

Answer 9

DRUGS COMPETING FOR THE SAME RECEPTOR SITE.
This may involve :

• agonist- antagonist interactions
• agonist – partial agonist interactions

Cross tolerance –
Altered receptor numbers or affinity due to one drug affecting the actions of another

Question 10

What are some examples of pharmacodynamic interactions? (4)

Answer 10

• Propranolol or atenolol interfere with the actions of ß-receptor agonists such as salbutamol or terbutaline
• Thiazide diuretics affect with plasma ion concentrations (reduce potassium) and enhance the actions of cardiac glycosides (digoxin)
• Sildenafil (Viagra) inhibits phosphodiesterase type V, raising the levels of cGMP. This can potentiate the action of drugs such as glyceryl trinitrate (used to treat angina) which activate guanylate cyclase. The combination can produce fatal hypotension
• Warfarin (anticoagulant, antagonizing vitamin K) can cause bleeding – exacerbated by aspirin (anti-inflammatory) which blocks the production of prostaglandins and can cause bleeding in the stomach