Common interactions

Question 1

What is the interaction between microgynon 30 and phenytoin?

Answer 1

COC interacts with many anti-epileptic drugs.

Reduce the efficacy of pregnancy prevention due to phenytoin inducing the metabolism of oestrogens.

Question 2

How is the interaction between COC and antiepileptic drugs managed?

Answer 2

Adjust dose of COC to at least 50 microgram daily and use tricycling regimen with four day pill free break.

Take OC for three consecutive cycles without a break, then have a four-day pill free interval.

Question 3

What is the interaction between amiodarone and digoxin?

Answer 3

Amiodarone reduces the excretion of digoxin.
Levels of digoxin will rise and toxicity can occur.
BNF and s
Stockleys recommend reducing digoxin doses by half.

Question 4

What is the interaction between amiodarone and warfarin?

Answer 4

Amiodarone inhibits the metabolism of warfarin through the inhibition of cytochrome p450 enzymes.

Need to monitor the INR and adjust the dose of warfarin accordingly.

Question 5

What is the interaction between clarithromycin and simvastatin?

Answer 5

Macrolides: alarm bells.
Clarithromycin is CYP inhibitor: increased levels of simvastatin and increased risk of myopathy.

Can either stop statin for duration of clarithromycin course + 3 days or we could change to metronidazole.

Question 6

What is the interaction between itraconazole and sildefanil?

Answer 6

azoles: CYP inhibitors.
Increased levels of viagra result.
Dose should be halved.

Question 7

What is the interaction between lithium and NSAIDs?

Answer 7

NSAIDs inhibit prostaglandin synthesis, this leads to reduced renal perfusion and reduced excretion of lithium.

Toxicity could result as lithium has a very narrow therapeutic window.

Question 8

When dispensing lithium what needs to be checked?

Answer 8

That it is the same brand the patient has had before as the therapeutic window is so narrow.

Question 9

What should be checked for methotrexate?

Answer 9

ONCE WEEKLY.

Can be used alongside NSAIDs.

Question 10

How should hypertension be treated in pregnancy?

Answer 10

Atenolol = no.
Labetolol = yes outside of first trimester.
Methyldopa also is fine.

Question 11

Why can quinolone antibiotics not be used in epileptics?

Answer 11

Quinolones lower the seizure threshold.

Question 12

Can trimethoprim be used in pregnancy?

Answer 12

Should be avoided in pregnancy.
Especially in 1st semester.
It has an antifolate effect and can lead to neural tube defects.

Question 13

Can metoclopramide be used in parkinsons?

Answer 13

No.

Question 14

Why can metoclopramide not be used in Parkinsons?

Answer 14

Induces acute dystonic reactions.
Should be changed to domperidone 10mg tablets 1 tds.
Domperidone does not cross BBB.

Question 15

What occurs when tramadol are used as well as SSRIs?

Answer 15

Increased risk of CNS toxicity.

SSRIs and tramadol also lower the seizure threshold.

Question 16

Why can erythromycin and domperidone not be taken at the same time?

Answer 16

Erythromycin increases domperidone levels 3x through inhibition of CYP3A4 and also there is prolongation of the QT interval.

Question 17

For who are ACEI not recommended?

Answer 17

Afro-caribbean patients because they have lower renin levels.

Therefore, less responsive.
CCBs are better.

Question 18

Why can Calcichew D3 forte not be used in renal failure patients?

Answer 18

Calcium salts are used as a phosphate binder in renal patients. Calcichew D3 forte contains colecalciferol which requires hydroxylation by the kidney. This conversion is limited in patients with renal impairment. Calcichew should be used.

Question 19

What should not be dispensed due to the possibility of mechanical trauma to fetus?

Answer 19

Applicator with canestan pessary.