Christchurch Medicines Information Service

Infant exposure to antidepressants via breast milk

All antidepressants transfer into milk to some degree. The extent of infant exposure depends on several factors: maternal exposure (plasma concentration), distribution of the antidepressant into milk, volume of milk ingested and infant clearance (which increases with age).
The most accurate measure of exposure is infant plasma concentrations; however, these are rarely reported. Studies more commonly measure drug concentration in milk to estimate the infant’s ‘dose’. This is usually expressed as a percentage of the weight-adjusted maternal dose or the relative infant dose (RID):

Drugs with a RID or infant plasma concentration (relative to maternal) below 10% are generally considered safe in breastfeeding. However, more caution is required if:

• the infant is premature
• the mother’s dose is very high or she is on multiple psychotropic drugs
• the drug is highly toxic (not a major concern with the antidepressants currently available in New Zealand)

Pharmacokinetic considerations

Infant exposure is lower for drugs with a low fractional oral bioavailability (F). This is because less of the drug ingested via milk reaches the infant’s systemic circulation.

Key prescribing points

• Switching antidepressants solely due to breastfeeding is not recommended. Women who have been taking an effective antidepressant while pregnant should generally continue on the same drug. In utero exposure is much greater than exposure via milk, and switching post-partum may increase the risk of relapse.
• For initiation post-partum, firstly consider any previous response to antidepressants. SSRIs have the most safety data in breastfeeding and no serious adverse effects have been observed. Choose one with a low RID if possible.
• Use the lowest effective dose.
• Reducing infant exposure by taking the dose immediately after breastfeeding is sometimes recommended, but there is little evidence to support this, and it may make breastfeeding more difficult for the mother.
• Monitor the infant for possible adverse effects (see table), recognising that these may be hard to distinguish from a simply ‘fussy’ baby.

Need further help?

• Search the LactMed® database for detailed information on specific drugs in breastfeeding.
• Contact Medicines Information on 03 364 0900 or email.

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