Hi Wendy
There is good evidence of harm coming to babies who are fed artificial substitutes for breastmilk.  There is evidence of no harm coming to babies who are breastfed by heavy cannabis users.  Frankly, I would be more concerned about a mother who is a heavy cannabis user being able to make up a formula feed safely.  It is a task that I have found challenging when completely sober.  
It seems to me that the IBLCE Code of Ethics and the ILCA Scope of Practice make it quite clear that an IBCLC is obliged to provide this mother with the available evidence and to support her decision to breastfeed her baby.  Of course, it will be important to encourage her to ensure that there is always an adult around who is in a fit state to care for the baby ... but let's remember that many addicts are quite functional and work with the mother who presents rather than the stereotype we might have in our heads.
Cheers
Nina Berry
Australia

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