Drug choice and prescribing in special cases

Question 1

WHO criteria for good prescribing

Answer 1

1. Define patients problem
2. Specify the therapeutic objective-what do you want to achieve with the treatment?
3. Verify suitability of your proposed treatment
4. Start
5. Give information instructions and warnings
6. Monitor, potentially stop

Question 2

Prescribing in renal impairment

Answer 2

Renal impairment may reduce elimination of renally excreted drugs, leading to accumulation/toxicity

Question 3

MEasure for renal function

Answer 3

Creatine clearance or eGFR:

eGFR:

• >90 ml/min/1.73m^2 normal renal function
• 89-60 Mild impairment
• 30-59 Moderate
• 15-29 Severe
• <15 Established renal failure

Question 4

Special considerations/tips for renal impairment

Answer 4

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 5

Cosniderations for prescribing in pregnacny

Answer 5

• Almost all drugs cross placents
  
  • Heparin is excpetion( high MW)
• So avoid drugs, unless benefits outweigh harm
• ASSUME every woman of child bearign age is pregnant until you know otherwise

Question 6

Drugs that are especially problematic in pregnancy

Answer 6

• Anti-epileptics
• Anticoagulants
• Antibiotics
• Antihypertensives
  
  • Labetol, nifedipine, methyldopa
• Antidiabetics; insulin
  
  • Metformin, glibenclamide
• Antidepressants

Question 7

Side effects of anti-epileptics in pregnancy(Phenytoin, valporate, carbamezapine)

Answer 7

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 8

Management of epilepsy in pregnancy

Answer 8

Continuation of drugs preffered, or planned dicontinuation

• Carbamezapine previously preffered w/5mg folic acid to reduce neural tube defects

Lamotrigine now first line in generalised tonic-clonic to avoid teratogenic/interactign drugs

Question 9

Interactions with oral contraceptives

Answer 9

• 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 10

Anticoagulants in pregnancy

Answer 10

Warfarin is teratogenic

• chondroplasia punctata (altered bone growth)
• optic atrophy
• mental retardation

Avoid warfarin in trimester 1 and 3

Favour LMWHs

Question 11

Presribing in hepatic impairment

Answer 11

BNF: LFTs are a poor guide

Considerations:

• Hepatic clearance
• Protein binding
• Sodium retention
• Effects on coagulation (INR may be increased)
• Gastric effects
• CNS effects
• Sedation