Prescribing in special groups

Question 1

when is it high risk to prescribe drugs in pregnancy

Answer 1

1st trimester - organ formation occurs 3-8 weeks, affected by teratogens

Question 2

change to volume of distribution in pregnancy

Answer 2

blood volume increases –> increase volume of distibutions –> need to increase the dose of some drugs e.g. lamotrigine

Question 3

Elimination of drugs in pregnancy

Answer 3

enhanced renal elimination of some drugs –> higher dose needed e.g. LMWH, antiepileptic

Question 4

Effect of drugs in 2nd and 3rd trimester

Answer 4

can affect growth (IGUR) or have toxic effects on tissues

Question 5

Prescribing in pregnancy

Answer 5

When prescribing in pregnancy one should assume that all drugs will cross the placenta unless they have very high molecular weight (e.g. heparins) - only prescribe drugs if really necessary

Question 6

Drugs to be avoided in pregnancy

Answer 6

ACE inhibitors
Aminoglycosides 
Anrdrogens
Opiates 
Quinolone antibiotics 
Sodium Valproate
Tetracyclines 
Warfarin

Question 7

Drugs present in breast ilk that can cause harm to child

Answer 7

amidoarone 
antithyroid drugs
benzodiazepines
lithium salts 
radioactive iodine 
statins 
sulphnamides

Question 8

effects of dopamine agonists (carbegoline) on milk production

Answer 8

inhibits prolaction –> reduced milk production

Question 9

effects of dopamine anatagonists on milk production e.g. domperidone

Answer 9

stimulate prolactin –> increased milk production

Question 10

volume of distribution in kids

Answer 10

The younger the child, the greater their total body water as a % to their weight –> high volume of distribution –> low concentration of drug.

Question 11

water soluble drugs in kids

Answer 11

younger kids have a greater volume of distribution - higher doses per kg of bodyweight must be given to infants and children than to adults (decreasing steeply with age) of some drugs (e.g. gentamicin)

Question 12

Protien bound drugs in kids

Answer 12

Plasma proteins (e.g. albumin) reduced in neonates 
Reduced plasma-protein binding causes an increase in ‘free’ drug

Question 13

Excretion of drugs in kids

Answer 13

Is reduced as the kidneys are developing, and GFR is low.
Therefore, drugs excreted by kidneys can accumulate
It is important to adjust dosing regimens and monitor closely, particularly for drugs like gentamicin

Question 14

Drugs to avoid in children

Answer 14

· chloramphenicol (grey baby syndrome)
· aspirin (Reye’s syndrome)
tetracyclines (affects bone and teeth).

Question 15

Absorption of drugs in elderly

Answer 15

· Absorption is altered due to decreased gastric motility, increased pH and decrease emptying, reduced saliva, and decreased blood supply.
However, evidence suggests that doses do not need altering.

Question 16

Volume of distribution in elderly

Answer 16

increased proportion of fat therefore increase Vd of lipid soluble drugs therefore lipid soluble dugs will accumulate (e.g. diazepam).
Thus, lower doses of water soluble drugs can be required (e.g. loading doses of digoxin).

Question 17

Plasma protein binding in elderly

Answer 17

There is also reduced plasma-protein binding (i.e. concentration) this causes an increase in ‘free’ drug (e.g. phenytoin) this can lead to an increased risk of toxicity.

Question 18

Metabolism i elderly

Answer 18

First pass metabolism decreases due to reduced hepatic blood flow, this can significantly increase the bioavailability of lipid soluble drugs.
The overall hepatic metabolism with cytochrome P450 enzymes also decreases with age, meaning that metabolic clearance is reduced, resulting in increased levels or duration of action of drugs extensively metabolised (e.g. morphine) and pro-drugs may be less effective.

Question 19

Renal elimination in elderly

Answer 19

decreased renal elimination with age due to a reduction in GFR.
Renally excreted drugs need dose adjustment (digoxin, gentamycin, lithium, opiates)
Renal tubular function also falls with age. Elderly people are more sensitive to nephrotoxic drug damage.

Question 20

The geriatric giants

Answer 20

The commonest impairments in old age - immobility, instability, incontinence, and impaired intellect.
Diuretics make incontinence worse.
Certain drugs can cause confusion (anticholinergics, antidepressants, hypnotics).

Question 21

treatment for mild-moderate Alzheimer’s

Answer 21

Acetylcholinesterase inhibitor (e.g. donepizil)

Question 22

Effects of liver failure

Answer 22

• low proteins
• reduce clotting –> increased anticoagulant sensitivity (warfarin, NSAID)
• fluid overload - avoid drugs that exacerbate this (NSAID, corticosteroids)

Question 23

Drugs to avoid in liver failure

Answer 23

• rifampicin - cannot be excreted if biliary obstuction
• drugs that exacerbate fluid overload - NSAIDs, corticosteroids
• durgs that can preciptate hepatic encephalopathy - sedatives and diuretics

Question 24

Drugs to avoid in renal failure

Answer 24

• vancomycin, lithium, gentamycin (renally excreted)
• NSAIDs (nephrotoxic)
• bendroflumethiazide and nitrofurantoin (ineffective)
• metformin (increased risk of lactic acidosos)
• potassium sparing diuretics (hyperkalaemia)