A new Spanish study has found that breastfeeding mums on drugs or medicines can pass on the substances to their babies.
Scientists from various Spanish hospitals and research centres reviewed the methods used to detect substances in breast milk, their adverse effects, and the recommendations that mothers should follow.
"The general recommendation is to totally avoid drug abuse while breastfeeding, because these substances can pass directly through to the newborn," Oscar García Algar, co-author of the study and a doctor in the Paediatrics Department at the Hospital del Mar in Barcelona, tells SINC.
"This recommendation extends to the prenatal period, because these substances are passed on to the foetus via the placenta, and then in the postnatal period via the environment.
"If they have exposure through the milk, they will certainly also have had it during the pregnancy, and they can also be in the environment, as is the case with tobacco smoke," he said.
For their study, the team used the average daily intake of the breastfeeding baby, around 150 millilitres of milk per kilo of weight, as a benchmark.
The recommendations are listed for each substance, taking the advice of the American Academy of Pediatrics (AAP) as a reference.
The breast milk of smoking mothers contains between 2 and 240 nanograms of nicotine per millilitre, which means their babies receive a dose equivalent to 0.3 to 36 micrograms/kg/day. These infants tend to suffer more from colic and are more prone to respiratory infections.
The advice is to give up smoking during pregnancy and breastfeeding, or at least to limit the habit as much as possible, extend the time between the last cigarette and the baby's feed, use nicotine patches, smoke outside the house and avoid smoky environments.
Caffeine, found in coffee, tea, cola drinks and medicines, can cause irritability and insomnia.
Although the level of caffeine absorption varies greatly from one person to another, this substance has a lengthy half-life in newborns.
For this reason, it is recommended to reduce consumption during breastfeeding to a maximum of 300 mg/day, equivalent to around three cups of coffee per day.
For alcohol, the exact risk is still ill-defined, and no studies have been carried out to correlate the dose, although some research suggests it can harm the infant's motor development, as well as causing changes to their sleep patterns, reduce the amount they eat, and increase the risk of hypoglycaemia.
The AAP feels that alcohol consumption is compatible with breastfeeding, but this study states that no amount can be considered safe until the levels in breast milk are established.
Cannabis, which is transmitted both through the mother's milk and smoke, can cause sedation, lethargy, weakness and poor feeding habits in breastfeeding babies. The long-term risks are also unknown.
Women are advised not to use it, but if they use marijuana occasionally, the experts advise them to do so several hours before feeding, and not to expose their children to the smoke.
The advice on cocaine, meanwhile, is to "totally avoid it" during breastfeeding.
Women are also advised against breastfeeding if they take amphetamines. These can cause agitation, crying and lack of sleep. Using them also reduces a mother's ability to care for her children.
Breastfeeding is not recommended either for women who use heroin, which is excreted into the milk in sufficient amounts to cause addiction in the baby.
In the case of "need", the advice is to allow at least one or two days to pass after taking the drug before feeding the baby, and to start a substitute treatment as soon as possible, if possible with methadone.
Other opiates used as medicines - morphine, meperidine and codeine - are excreted into the milk in minimal amounts and are compatible with breastfeeding, as are benzodiazepines, as long as they are taken in controlled doses. These are the drugs most frequently prescribed to women during pregnancy and after birth.
In terms of anti-depressant and anti-psychotic drugs, the AAP says "these can be a cause for concern during breastfeeding". For now, their effects on breastfeeding babies are unknown, and further studies are recommended.
The findings have been published in this month's issue of the journal Analytical and Bioanalytical Chemistry.