Recombinant humanized IgG4k monoclonal antibody that blocks the human integrin alpha-4 subunit making it difficult for T lymphocytes to pass through the meninges. Its use is approved in the treatment of severe forms of multiple sclerosis (EMA 2017) and in Crohn's disease when resistant to other treatments (Yarur 2013, FDA 2012). Its very high molecular weight explains the small or zero transfer into milk observed (Hainke 2015, Baker 2015). Of three mothers in whom the concentration of natalizumab in breast milk was measured, detectable levels only appeared in two. With these measurements, expert authors have calculated a relative dose for the infant of no concern, around 1.7% or, if the maximum peak is considered, 5.3%. But in addition, its low oral bioavailability would hinder its transfer into the infant's plasma via ingested breast milk because, due to its protein nature, it degrades in the gastrointestinal tract, and is not absorbed (Cree 2013), except in premature infants and the immediate neonatal period where there may be greater intestinal permeability. Despite the low excretion in breast milk and the lack of intestinal absorption by the infant, due to the long elimination half-life and the limitation of the number of available reports, opinions among experts are divided between those who do not recommend it (Mahadevan 2017), those who think that it is probably compatible (Almas 2016, Baker 2015, Briggs 2015) and those who have not expressed an opinion due to lack of sufficient studies (Hainke 2015, Damas 2015, Cree 2013, Houtchens 2013, Yarur 2013, Mahadevan 2011) , with experts being more in favor of discouraging it in Europe than in the USA (Alroughani 2016). Other monoclonal antibodies similar to natalizumab, such as adalimumab, infliximab and certolizumab, are considered low risk and probably compatible with breastfeeding (Damas 2015, van der Woude 2010). See below the information of these related products: