Chapter 1: Basic principles of prescribing

Question 1

Enzyme inducers

Answer 1

increase P450 enzyme activity = reduced levels of other drugs

PC BRAS

• Phenytoin
• Carbamezapine
• Barbituates
• Rifampicin
• Alcohol (chronic excess)
• Sulphonylureas

+ smoking + griseofulvin

Question 2

Enzyme inhibitors

Answer 2

= reduce P450 enzyme activity = increased levels of other drugs

AO DEVICES

• Allopurinol
• Omepraxole
• Disulfiram
• Erythromycin
• Valproate
• Isoniazid
• Cirpofloxacin
• Ethanol (acute intaxication)
• Sulphonamides

+ metronidazole + cimetidine + amiodarone + imidazoles + SSRIs

Question 3

Drugs that would be DETRIMENTAL intraoperatively if discontinued

Answer 3

Calcium Channel Blockers

Beta Blockers

Which therefore must be continued

Question 4

Drugs which must be INCREASED during surgery

Answer 4

Long-term corticosteroids e.g. prednisolone, as adrenal atrophy so can’t mount adequate physiological stress response to surgery. (As with doubling of steroids with sick-day rules) During induction of anaesthesia, patients should be given IV steroids.

Question 5

Drugs that worsen seizure control in patients with epilepsy:

Answer 5

The following drugs may worsen seizure control in patients with epilepsy:

• mefenamic acid
• methylphenidate (used in ADHD)
• alcohol, cocaine, amphetamines
• ciprofloxacin, levofloxacin
• aminophylline, theophylline
• bupropion

*Some medications such as benzodiazepines, baclofen and hydroxyzine may provoke seizures whilst they are being withdrawn.

Other medications may worsen seizure control by interfering with the metabolism of anti-epileptic drugs (i.e. P450 inducers/inhibitors).*

Question 6

Drugs metabolised by P450s!

Answer 6

Drugs Metabolised: COWEST

• Ciclosporin
• OCP
• Warfarin
• Epileptic: phenytoin, carbamazepine
• Statins
• Theophyline

Question 7

What drugs should be stopped or altered before surgery?

Answer 7

Drugs to consider stopping/altering before surgery:

• Anticoagulants. See thromboprophylaxis. In summary, stop warfarin and swap to LMWH on admission.
• Antiplatelets such as aspirin should also be stopped
• Oral contraceptive pill / hormone replacement therapy should be stopped 4 weeks before major surgery / leg surgery.
• Lithium should be omitted the day before surgery.
• Insulin. See Pre-operative Planning. In summary, continue basal insulin at 80% usual dose and discontinue short-acting insulin and place patient on sliding-scale.
• Oral hypoglycaemics should generally be stopped before the day of the surgery until first post-op meal. Metformin may be continued if eGFR>60 and only missing one meal.
• Potassium sparing diuretics should be stopped the day of surgery to prevent hyperkalaemia.
• ACE inhibitors are stopped on the day of surgery as can cause hypotension during anaesthesia.