Pharmacology In Pregnancy And Breast Feeding

Question 1

What is the difference between pharmacokinetics and pharmacodynamics in pregnancy?

Answer 1

Pharmacokinetics
Absorption: oral may be more difficult, increase fsstric emptying and gut motility; IM may increase due to increased blood flow; inhalation may increase absorption due to increased CO and decreased tidal volume
Distribution: volume distribution increased due to increase in plasma volume, and increase in fraction of free drug due to dilution of plasma decreasing relative amount of plasma proteins
Metabolism: can be increased or decreased due to oestrogen and progestogens inducing or inhibiting liver enzymes
Excretion: GFR increases, meaning increased excretion

Pharmacodynamics
Less understood, may affect site of action and receptor response to drugs, efficacy may be different and adverse effects may be different

Question 2

What are the main concerns regarding the use of medication in pregnancy?

Answer 2

Placental transfer (depends on molecular weight, polarity and lipid solubility)
Teratogenicity (3-8wks)
Fetotixicity

Question 3

What is the main concern regarding the use of medication in breastfeeding?

Answer 3

Most drugs will be present at lower doses through breast feeding

Question 4

Where can information be found about risks with specific drugs in pregnancy/breastfeeding?

Answer 4

BNF
UK Tetrology Information Service
Schedule of product characteristics
Drugs and lactation database

Question 5

What are the principles when prescribing for these patients?

Answer 5

Prescribing in pregnancy
Avoid: retinoids, nsaids, ACEI, ARBs, anticonvulsants, anticoagulants, alcohol 
- non pharmacological first
- drug with best safety record
- check SPC
- lowest effective dose
- shortest time possible
- avoid in first 10 weeks if can
- consider stopping or reducing dose
- do not undertreat 

Prescribing in breastfeeding
Avoid: cytoxics, immunosuppressants, anti convulsants, drug abuse, amiodarone, radioiodine
- avoid unnecessary drug dose
- check up to date info
- if licensed and safe in paediatric use then likely safe in breast feeding
- choose drugs with pharmacokinetic properties which reduce infant exposure