Drug interactions

Question 1

What sex eliminates drugs quicker?

Answer 1

Females tend to eliminate drugs quicker

however, drugs tend to distribute differently in female

Question 2

What is a pharmacodynamic drug interaction?

Answer 2

Pharmacodynamics is how the drug effects the body (very loosely).

• interactions of this type amplify or negate the drugs effect
• act at the receptor, tissue, or organ level
• are dependant oh the drugs half lives

Question 3

What drug-drug and drug-food interactions must be considered if prescribing ACE-i?

Answer 3

ACE-i increases the likelihood of becoming hyperkalaemic

• potassium sparing diuretics such as spironolactone and amiloride further increase risk
• patients who have swapped to a low salt diet using - lo-salt (potassium containing salt subsititue)

Question 4

What is a pharmacokinetic drug reaction?

Answer 4

• pharmacokinetics broadly is how the body effects the drug
• so interactions = how one drug/substance affects the biotransformation of another drug
• biotransformation = the conversion of the drug into metabolites to makes them more waters soluble

Question 5

What are the two types of pharmacokinetic drug interactions?

Answer 5

Induction & inhibition (of removal)

• Induction = accelerating the removal of a drug, reducing/abolishing efficacy of intended action, timeframe = days to week
• inhibition = deceased removal of drug, accumulation of the active drug leads to off-target effects and toxicity, timeframe = hours to days

Question 6

What does one have to consider when prescribing rifampicin?

Answer 6

It is an CYP450 enzyme inducer that reduces the efficacy of progesterone’s and oestrogens.
Ask about oral contraception when prescribing rifampicin.

Question 7

When giving 2 negative inotropes what is the drug interaction? name some negative ionotropes.

Answer 7

pharmacodynamic
beta-blockers - propanolol, bisoprolol
CCB - verapamil, amlodipine, nefedipine

Question 8

How do macrolides (clarithromyocin, erythromyocin) effect warfarin metabolism?

Answer 8

macrolides inhibits the same enzyme that metabolises warfarin increases circulating warfarin and increases INR.

Question 9

What are examples aminoglycosides? What are examples of loop diuretics? What drug interaction happens between them?

Answer 9

aminoglycosides - gentamicin & streptomicin
loop diuretics - furosemide & bumetanide
They’re both ototoxic and have a pharmacokinetic drug interaction - additive effect

Question 10

How are lithium salts excreted? How does this relate to it’s drug interaction with ramipril?

Answer 10

• lithium salts are excreted via the kidney
• ACE-i (ramipril) reduce eGFR and increase sodium loss
• this causes a retention of lithium salts increasing the likelihood of toxicity.

Question 11

How does warfarin interact with NSAIDs?

Answer 11

• NSAIDs increased risk of gastro bleeding due to inhibition of COX-1 enzyme (creates gastro protective protein)
• Warfarin increases bleeding risk 10 fold though not affecting the concentration of warfarin

Question 12

How does carbamazepine affect the the metabolism of levenorgestrel (the morning after pill)?

Answer 12

-need to take double the dose as carbmazepine is CYP enzyme inducer that increases the metabolism of the drug

Question 13

What are CYP inhibitors?

Answer 13

• anti-fungals -azoles
• erythromycin
• grapefruit juice
• cranberry juice

Question 14

What are the strongest CYP inducers?

Answer 14

• rifampicin
• st. johns wort
• first gen AED (phenytoin, carbemazepine)