Interactions

Question 1

Name the enzyme inhibitors (SICKFACES.COM)

Answer 1

Sodium valproate
Isoniazid / itraconazole
Cimetidine
Ketoconazole
Fluconazole / fluoxetine
Alcohol (acute, binge) / Amiodarone
Chloramphenicol
Erythromycin + clarithromycin
Sulphonamides (co-trimoxazole)
Ciprofloxacin
Omeprazole
Metronidazole

Also:
Grapefruit juice

Question 2

Name the enzyme inducers (SCRAP GPSS)

Answer 2

Sulphonylureas
Carbamazepine
Rifampicin
Alcohol (chronic)
Phenytoin

Griseofulvin
Phenobarbital
St John’s Wort
Smoking

Question 3

What are the main interactions with amiodarone?

Answer 3

1. Amiodarone inhibits warfarin metabolism- enhanced anticoagulant effect
2. Increased risk of bradycardia, AV block, myocardial depression with beta blockers
3. Risk of ventricular arrhythmias with lithium
4. Plasma concentration of digoxin increased by amiodarone

Amiodarone has a very long half life so there is potential for drug interactions to occur weeks/months after stopping treatment

Question 4

What are the common interactions with digoxin?

Answer 4

1. Plasma conc of digoxin increased by amiodarone (enzyme inhibitor)
2. Plasma conc of digoxin increased by erythromycin (enzyme inhibitor)
3. . Plasma conc of digoxin reduced by rifampicin (enzyme inducer)
4. Plasma conc of digoxin reduced by St John’s Worst (enzyme inducer)
5. Increased toxicity of digoxin if hypokalaemia occurs with loop and thiazide diuretics
6. Plasma conc of digoxin increased by CCBs

Question 5

What are the common interactions with lithium?

Answer 5

1. Risk of lithium toxicity with ACEi (excretion reduced)
2. Risk of lithium toxicity with NSAIDs (excretion reduced)
3. Sodium depletion with loop and thiazide diuretics (excretion of lithium reduced)
4. Risk of ventricular arrhythmias with amiodarone

Question 6

What are the common interactions with methotrexate?

Answer 6

1. Increased risk of infection with vaccines
2. PPIs at high doses reduce clearance of methotrexate increasing risk of toxicity
3. Penicillins increases risk of methotrexate toxicity
4. Trimethoprim- both folate antagonists, increased risk of side effects and nephrotoxicity

Question 7

What are the common interactions with phenytoin?

Answer 7

1. Effects of phenytoin enhanced by NSAIDs
2. Amiodarone inhibits phenytoin metabolism
3. Phenytoin accelerates metabolism of warfarin
4. Cimeditine inhibits metabolism of phenytoin
5. Plasma conc of phenytoin increased by fluoxetine
6. St John’s Wort reduces plasma conc of phenytoin
7. Ciprofloxacin affects the concentration of phenytoin
8. Decreases efficacy of combined contraceptive pill
9. Phenytoin decreases exposure to NOACS

Question 8

What are the common interactions with theophylline?

Answer 8

1. Increased risk of convulsions with quinolones e.g. ciprofloxacin
2. Plasma conc of theophylline reduced by St John’s Wort
3. Plasma conc of theophylline reduced by rifampicin
4. Plasma conc of theophylline increased by cimetidine
5. Plasma conc of theophylline increased by fluconazole
6. Smoking can increase theophylline clearance and increased doses of theophylline are therefore required

Question 9

What are the common interactions with warfarin?

Answer 9

1. Anticoagulant effect increased by NSAIDs
2. Anticoagulant effect increased by fluconazole
3. Anticoagulant effect increased by statins
4. Anticoagulant effect increased by ciprofloxacin, erythromycin, metronidazole
5. Anticoagulant effect reduced by griseofulvin
6. Anticoagulant effect reduced by antiepileptics
7. Alcohol effects anticoagulant control
8. Anticoagulant effect antagonised by Vitamin K
9. Anticoagulant effect enhanced by cranberry juice

Question 10

What is the risk of consuming tyramine based food and drink e.g. cheese if on MAOIs?

Answer 10

Hypertensive crisis

Question 11

How does alcohol interact with TCAs and mirtazapine?

Answer 11

Increased sedative effect

Question 12

What are the main interactions with combined oral contraceptives?

Answer 12

• Enzyme inducing drugs increase metabolism of contraceptives. Additional contraceptive precautions should be taken for 4-8 weeks after stopping treatment
• Some ABX may reduce efficacy of the pill by impairing bacterial flora responsible for recycling ethinylestradiol e.g. ampicillin, amoxicillin, doxycycline. Additional precautions are required for duration of treatment and for 7 days after stopping

Question 13

What are the main interactions with progesterone only contraceptives?

Answer 13

Efficacy reduced by enzyme inducers

Additional protection is needed for duration of treatment and 4 weeks after

Question 14

What are the main interactions with sympathomimetics e.g. pseudoephedrine?

Answer 14

• MAOI- hypertensive crisis

- Beta blockers- hypertension risk

Question 15

What are the main interactions with Orlistat?

Answer 15

• Orlistat reduces plasma conc of amiodarone
• Anticoagulants- monitor
• Acarbose for diabetes due to its GI effects
• Reduces absorption of ciclosporin

Question 16

What is a pharmacokinetic interaction?

Answer 16

These occur when one drug alters the absorption, distribution, metabolism, or
excretion of another drug, thus increasing or reducing the amount of drug available
to produce its pharmacological effects.

Question 17

What is a pharmacodynamic interaction?

Answer 17

This is where effects of one drug are changed by the presence of another drug at its
pharmacological site of action.

e.g. electrolyte imbalance, combined toxicity, antagonising effects

Question 18

What PPI does clopidogrel interact with and what would be an alternative?

Answer 18

Omeprazole and esomeprazole

Lansoprazole would be an alternative