Drug Choice and Prescribing in Special Cases

Question 1

What are the 6 steps of WHO good prescribing?

Answer 1

Step 1: Define the patient’s problem
Step 2: Specify the therapeutic objective
- What do you want to achieve with the treatment?
Step 3: Verify the suitability of your P-treatment
- Check effectiveness and safety
Step 4: Start the treatment
Step 5: Give information, instructions and warnings
Step 6: Monitor (and stop?) treatment

Question 2

What is the issue associated with prescribing in renal impairment?

Answer 2

BNF –
Some but not all drugs are renally excreted
Renal impairment may reduce elimination of these drugs leading to accumulation / toxicity.

Question 3

What are the 2 main ways to measure renal function?

Answer 3

• creatinine clearance
• eGFR (mainly, for dosage)

Question 4

What are the eGFR ranges?

Answer 4

eGFR > 90 mL/min/1.73m2 : Normal renal function
eGFR 89-60 mL/min/1.73m2: Mild renal impairment
eGFR 30-59 mL/min/1.73m2: Moderate renal impairment
eGFR 15-29 mL/min/1.73m2: Severe renal impairment
eGFR < 15 mL/min/1.73m2: Established renal failure

Question 5

What are the alternate approaches to renal impairment?

Answer 5

Choose short acting agents (e.g. tolbutamide as a choice sulphonylurea)
Gentamicin – increase the dosage interval in renal impairment
Choose non-renally excreted alternatives
E.g. amlodipine in hypertension
Gliclazide in 2DM
Some drugs must be avoided in renal impairment
e.g. metformin
Some drugs require renal excretion to act may become ineffective in renal impairment
Thiazide diuretics

Question 6

How does pregnancy affect prescribing?

Answer 6

• avoid drugs unless benefits outweigh harm
• almost all drugs cross placenta

Question 7

What is the only drug that does not cross the placenta and why?

Answer 7

• heparin
• due to its large molecular weight and polarity

Question 8

What must you assume of every woman of child bearing age?

Answer 8

is pregnant

Question 9

What drug groups cause issues during pregnancy?

Answer 9

long term drugs
Anti-epileptics
Anticoagulants

Question 10

What drugs can be used safely using pregnancy?

Answer 10

• certain antibiotics
  -Antihypertensives - labetalol, nifedipine, methyldopa
• Antidiabetics – insulin
  Metformin, glibenclamide
• Antidepressants

Question 11

What are the anti-epileptic drugs associated with in pregnancy?

Answer 11

Phenytoin
- craniofacial abnormalities
- hypoplasia of distal phalanges
- growth deficiency
- mental deficiency
Valproate
- associated with neural tube defects
Carbamazepine
- similar to phenytoin but decreased risk

Question 11

What is the plan for anti-epileptic drugs during pregnancy?

Answer 11

Continuation of treatment is preferable - counselling
Or planned discontinuation
Carbamazepine was preferred
5mg folic acid given to reduce chances of neural tube defect
Lamotrigine used first line in generalised tonic-clonic seizures to avoid teratogenic / interacting drugs

Question 12

What are some interactions with oral contraceptives?

Answer 12

Inducing agents can lead to a failure of therapy
AEDs: phenytoin, carbamazepine and phenobarbital are inducers
Favour non-inducing agents or use alternative contraceptive methods
Rifampicin is a power inducer

Question 13

How do anticoagaulation affect pregnancy?

Answer 13

Warfarin is teratogenic
chondroplasia punctata (altered bone growth)
optic atrophy
mental retardation
Avoid warfarin in trimester 1 and 3
Favour LMWHs