I am sure that no one has ever done a study on a mother who took LSD
and then took samples of her milk.

LSD has powerful hallucinogenic effects with small amounts, but there
is no doubt that the amount that would get into the milk is tiny.
Hale says the half life is 3 hours, so that after 15 hours, there
would be almost none left in the mother's body and the amount in the
milk would always probably be extremely small, even too small probably
to cause any effects on the baby.  But that is theory and I have not
ever heard of a mother taking LSD and then breastfeeding, though I am
sure it happens..

Jack Newman, MD, FRCPC