From what I've read, you should avoid the intake of cannabinoids while breast feeding. They can be transmitted to your baby through the milk.. also avoid it while you're pregnant too.

BUT CHECK THIS OUT...............

Breast milk already contains natural cannabinoids!

Babies need to eat(drink) a large amount. Thanks to nature, the "endogenous cannabinoids" (endocannabinoids) in the breast milk will give your little one the munchies which will encourage it's appetite.

from Cannabinoids - Wikipedia, the free encyclopedia
The endocannabinoid 2-AG has been found in bovine and human maternal milk.
and Breast milk - Wikipedia, the free encyclopedia
Mother's milk has been shown to supply a type of endocannabinoid (the natural neurotransmitters which marijuana simulates), 2-Arachidonoyl glycerol.
Press About Marni Jackson - Pain, the fifth vital sign
The same substance in cannabis that can help people with multiple sclerosis deal with musle spasms, or cancer patients cope with nausea, is present in human breast milk too. Endogeneous cannabinoids, as they're known, are part of the body's own painkilling resources -cousins to more familiar ones like endorphins. Receptors for cannabinoids, located in the brain, spine and peripheral nerves, have just been discovered in the past decade.
The endocannabinoid signaling system: a potential target for next-generation therapeutics for alcoholism
Endocannabinoids are present in peripheral tissues as well as in the brain, and recently they were found to be present in breast milk.