6 General adult psychiatry6.2 Pregnancy, childbirth and the menstrual cycle
6.2.4 Drug handling in pregnancy and breastfeeding
2 min read
Written by Resident Doctor Committee · Published
New in the 2027 syllabus
The College added this topic for exams from January 2027.
Why exposure changes over time
Two ideas help here.
Pharmacokinetics is what the body does to the drug: absorption, distribution, metabolism and elimination. Pregnancy changes the body's physiology, so the same dose can give a different drug concentration.
Pharmacodynamics is what the drug does at a given concentration. Treat the effects on the mother and the vulnerability of the fetus or infant as separate questions.
Risk assessment weighs exposure to treatment against the risk of relapse and of untreated illness. Base it on the actual drug, the dose, the timing and the individual's own history.
Change in pregnancy | What it can mean |
|---|---|
Plasma volume and body water increase | Water-soluble drugs are distributed differently |
Albumin concentration falls | Protein binding changes, so the total concentration may not reflect exposure to unbound (free) drug |
The kidneys filter more | Drugs eliminated by the kidneys, such as lithium, are cleared faster |
Liver enzymes change | Clearance rises or falls, depending on the drug |
These changes reverse after birth | A dose adjusted for pregnancy may give higher concentrations postpartum |
Lithium and lamotrigine
Lithium needs specialist monitoring. Renal clearance, hydration and the changes around delivery all affect the level and the risk of toxicity.
Lamotrigine clearance often rises in pregnancy and falls after birth. Let symptoms, the individual's own baseline concentrations and specialist advice guide dose and monitoring.
If a dose has gone up in pregnancy, plan the postnatal review before delivery. Do not assume the pregnancy dose should carry on unchanged.
A drug level only makes sense if you know the sampling time, the time of the last dose, adherence, the clinical response and the person's physical state.
Breastfeeding
How much drug passes into milk depends on the drug's properties, the mother's concentration and the feeding pattern. A drug being present in milk does not by itself mean it is toxic.
Infants clear drugs less efficiently, especially if they are premature or unwell. The same exposure can therefore have different consequences.
Monitor the infant for poor feeding, sedation, irritability, poor weight gain and effects specific to the drug. Arrange laboratory tests when specialist advice says so.
When choosing treatment, consider what has worked before and the risk of relapse, as well as milk transfer and whether monitoring the infant is practical.
Planning and communication
Abrupt changes to medication may destabilise the illness. Record an individual plan, made with maternity and perinatal specialists where appropriate.
Check the current reproductive restrictions and safety advice for the specific medicine, particularly valproate and topiramate.
Shared records should state the dose plan, who is responsible for monitoring, the delivery arrangements, and what the parent should do if symptoms or signs of toxicity appear.