This is what I found:
The acetaminophen (Tylenol®) component of Percocet passes through breast milk, but it is not likely to cause problems (see Tylenol and Breastfeeding). However, the oxycodone component is a narcotic and could cause significant problems in a nursing infant. Percocet is often used to treat pain after delivering a baby, and many healthcare providers are comfortable recommending it for this use. However, it is a good idea to watch for any changes in your baby, such as breathing problems, excessive sleeping, or feeding problems, if you take Percocet while breastfeeding.
If you obey all the rules, you miss all the fun. - Katharine Hepburn