I've seen a bunch of posts lately on this so I thought I'd explain why it's different from alcohol or medication.

Alcohol

Cycles out of your milk like it does out of your blood. So once you're sober, you're ok to BF. The alcohol content of your blood is (for our purposes) the same as the content of your milk.

Medication

The amount of time it's in your blood depends on the drug (the "half life")

Weed

THC is stored in your fat cells for a long time (weeks to months or longer). Breastmilk is, in part, made from fat circulating in your blood. So as it's made, each new milk supply has THC in it. It doesn't cycle out.