drug interactions Flashcards
why shouldn’t you combine an aminoglycoside and a loop diuretic?
bonus question what type of reaction (PK/PD) is this…
can both cause ototoxicity
therefore a combined effect is a pharmacodynamic effect
acts directly on tissues etc..
what happens if somebody taking atorvastatin is put on rifampicin
can lead to a 7-fold decrease?? in atorvastatin concentration
name some of the most common CP450 enzyme inducers
inducers increase enzyme activity therefore decrease drug concentration
PC BRAS Phenytoin Carbemazepine Barbiturates Rifampicin Alcohol (in chronic XS) Sulfonylureas
name some CP450 enzyme inhibitors
(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
which herbal medicines may increase the bleeding risk of somebody on warfarin
Garlic (inhibits platelet aggregation)
Glucosamine (increases INR by interacting with warfarin)
give 3 examples of what effect St John’s Wort can have on certain medications
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…
when prescribing digoxin what should patients be told not to take supplements of / eat large amounts of
Liquorice
can cause hypokalaemia - digoxin can be toxic in hypokalaemia (compete for the same ion channel into cells) - and can also cause hypoK+
who shouldn’t take ginseng
people on oral hypoglycaemics (may cause hypoglycaemia)
which patients are more susceptible to harm from drugs (think of the acronym)
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)
which drugs shouldn’t you give to someone with mysathenia gravis?
- aminoglycosides (interfere with NM transmission) - SO NO GENTAMICIN
- Magnesium and magnesium containing preparations..
- DON’T CHANGE THEIR STEROID DOSE
nb - symptoms can also be exacerbated by opiates and B-blockers…
clozapine:
- how long worth of doses does a patient need to have missed to need re-titrating up from a low dose?
- what are the consequences if clozapine is restarted too quickly?
- 48hrs - then need to restart from low
- orthostatic hypotension can result - cardiovascular dysfuntion and shock are a risk - especially when co-prescribed with an anti-HTN agent
simvastatin + amlodipine
what is the max dose of simvastatin when prescribed concomitantly with amlodipine?
20mg
give a good mnemonic to remember the common ommissions in a drug history
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..
good mnemonic for a checklist for surgical patients.
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
what colour are the different strnegths of warfarin? 500 microgram 1mg 3mg 5mg
500 microgram - white
1mg - brown
3mg - blue
5mg - pink
why buy blue pants?
LITHIUM:
- should you prescribe by generic or brand name??
- what are the formulations?
- what are the monitoring requirements for lithium?
- prescribe by brand name
- 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 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
what electrolyte abnormalities can exacerbate Li and Dig toxicity?
Lithium - hyponatraemia
(IE diuretics and ACEis and NSAIDs can exacerbate Li toxicity)
Digoxin - hypokalaemia
what are the monitoring requirements for methotrexate
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
what can predispose patients to methotrexate toxicity?
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
what should you do if a patient has ?bone marrow suppression from methotrexate toxicity?
- STOP methotrexate
- Consult a haematologist/toxicologist
- 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.
ALWAYS CHECK WHEN PRESCRIBING TO SEE IF A DRUG INTERACTS WITH METHOTREXATE - LOTS OF COMMON ONES DO AND CAN PRECIPITATE TOXICITY / HAEMATOLOGICAL TOXICITY
also don’t give trimethoprim as it is a folate antagonist.
what acute illnesses should methotrexate be omitted in?
AKI
Infection
if in doubt - contact the specialist nurses!!!
