drug interactions

Question 1

why shouldn’t you combine an aminoglycoside and a loop diuretic?

bonus question what type of reaction (PK/PD) is this…

Answer 1

can both cause ototoxicity

therefore a combined effect is a pharmacodynamic effect
acts directly on tissues etc..

Question 2

what happens if somebody taking atorvastatin is put on rifampicin

Answer 2

can lead to a 7-fold decrease?? in atorvastatin concentration

Question 3

name some of the most common CP450 enzyme inducers

Answer 3

inducers increase enzyme activity therefore decrease drug concentration

PC BRAS
Phenytoin
Carbemazepine
Barbiturates
Rifampicin
Alcohol (in chronic XS)
Sulfonylureas

Question 4

name some CP450 enzyme inhibitors

Answer 4

(Inhibitors - decrease enzyme activity - inrease drug conc)

AODEVICES

Allopurinol
Omeaprazole (and some other PPIs..?)
Disulfiram
Erythromicin (macrolide ABx)
Valproate
Isoniazid
Ciprofloxacin (fluoroquinolones ie. a -floxacin)
Ethanol (acute intox)
SUlphonamides

Question 5

which herbal medicines may increase the bleeding risk of somebody on warfarin

Answer 5

Garlic (inhibits platelet aggregation)

Glucosamine (increases INR by interacting with warfarin)

Question 6

give 3 examples of what effect St John’s Wort can have on certain medications

Answer 6

Antidepressants - can massively increase the risk of toxicity effects - serotonin syndrome with SSRIs and hypertensive crises with MAOIs

Warfarin - reduces INR

COCP - reduces concentration of oestrogen and progestogens - reducing contraceptive cover…

Question 7

when prescribing digoxin what should patients be told not to take supplements of / eat large amounts of

Answer 7

Liquorice
can cause hypokalaemia - digoxin can be toxic in hypokalaemia (compete for the same ion channel into cells) - and can also cause hypoK+

Question 8

who shouldn’t take ginseng

Answer 8

people on oral hypoglycaemics (may cause hypoglycaemia)

Question 9

which patients are more susceptible to harm from drugs (think of the acronym)

Answer 9

GRASPED
genetics
reactions (ie check for allergies!!)
age (elderly and children)
sex
pregnancy(and other physiological states)
exogenous factors (foods and other drugs etc..)
disease states (eg. renal dys)

Question 10

which drugs shouldn’t you give to someone with mysathenia gravis?

Answer 10

1. aminoglycosides (interfere with NM transmission) - SO NO GENTAMICIN
2. Magnesium and magnesium containing preparations..
3. DON’T CHANGE THEIR STEROID DOSE

nb - symptoms can also be exacerbated by opiates and B-blockers…

Question 11

clozapine:

1. how long worth of doses does a patient need to have missed to need re-titrating up from a low dose?
2. what are the consequences if clozapine is restarted too quickly?

Answer 11

1. 48hrs - then need to restart from low
2. orthostatic hypotension can result - cardiovascular dysfuntion and shock are a risk - especially when co-prescribed with an anti-HTN agent

Question 12

simvastatin + amlodipine

what is the max dose of simvastatin when prescribed concomitantly with amlodipine?

Answer 12

20mg

Question 13

give a good mnemonic to remember the common ommissions in a drug history

Answer 13

DRUGS
Doctor - prescibed medications
Recreational - Tobacco / illicit / alcohol
User-acquired (OTC / herbal)
Gynae (oral contraceptive pill / depot / HRT)
Sensitivities and the nature of the reaction..

Question 14

good mnemonic for a checklist for surgical patients.

Answer 14

CASES
contraception - pregnancy in female patients / risk of thromboembolism
anticoagulation - risk of bleeding / need for decision about periop bridging / continuation
steroids - ? increase in steroids needed to prevent addisonian crisis peri-op?
ethanol - alcohol withdrawal risk / interactions with anaesthetic
smoking - ?lung disease

Question 15

what colour are the different strnegths of warfarin?
500 microgram
1mg
3mg
5mg

Answer 15

500 microgram - white
1mg - brown
3mg - blue
5mg - pink

why buy blue pants?

Question 16

LITHIUM:

1. should you prescribe by generic or brand name??
2. what are the formulations?
3. what are the monitoring requirements for lithium?

Answer 16

1. prescribe by brand name
2. lithium citrate (oral liquid) or lithium carbonate (oral tablet - about 2.5x as strong..)

liquid formulations of Li tend to be given BD

nb - shouldn’t switch between brands as the bioavailabiltiy changes

3. 3 MONTHLY checks of serum lithium conc 9can go out to 6 months if on stable dose for >1yr)? -
   DO AT LEAST 12hrs post dose and DOCUMENT HOW LONG POST THE DOSE

6 MONTHLY checks of:
Cardiac function - (do an ECG)
Thyroid function - (check for hypothyroidism)
Renal function - (can accumulate in renal dysfunction, and long term can cause renal impairment)
Calcium - can cause hypercalc
BMI - can cause weight gain
NO NEED FOR LFTS

Question 17

what electrolyte abnormalities can exacerbate Li and Dig toxicity?

Answer 17

Lithium - hyponatraemia
(IE diuretics and ACEis and NSAIDs can exacerbate Li toxicity)
Digoxin - hypokalaemia

Question 18

what are the monitoring requirements for methotrexate

Answer 18

WHAT DO THEY NEED?:
LFTs, Renal function, FBCs, CXR

do all as baseline before starting

repeat LFTs, Renal function, FBCs: (and ESR??)
every 1-2wks after starting therapy until stabilised then every 2-3 months after

Question 19

what can predispose patients to methotrexate toxicity?

Answer 19

Hypoalbuminaemia

Folate deficiency

Concomitant drug treatments prescribed that reduce the elimination of methotrexate (e.g. NSAIDs)

Ascites or effusions which can act as a depot as methotrexate distributes well into tissue fluid

Question 20

what should you do if a patient has ?bone marrow suppression from methotrexate toxicity?

Answer 20

1. STOP methotrexate
2. Consult a haematologist/toxicologist
3. Give folinic acid rescue therapy

Patients are likely to be neutropenic and so may also be suffering or be at risk of infection. Consult your local Trust guidelines about folinic acid rescue therapy and neutropenic sepsis.

Question 21

ALWAYS CHECK WHEN PRESCRIBING TO SEE IF A DRUG INTERACTS WITH METHOTREXATE - LOTS OF COMMON ONES DO AND CAN PRECIPITATE TOXICITY / HAEMATOLOGICAL TOXICITY

Answer 21

also don’t give trimethoprim as it is a folate antagonist.

Question 22

what acute illnesses should methotrexate be omitted in?

Answer 22

AKI
Infection

if in doubt - contact the specialist nurses!!!