Women who are breastfeeding need to be particularly careful about what medications they take or use topically, since these can end up in breastmilk. You don't want to be accidentally medicating your baby, especially since even small doses of most drugs can be dangerous to children because of their smaller size. Salicylic acid is one drug you should speak with your doctor about.
Salicylic acid is used topically to treat a number of different skin conditions, including acne, calluses, corns, dandruff, psoriasis, seborrheic dermatitis and both common and plantar warts. It causes the outer layers of skin to peel, and is absorbed into the skin. It is included in both over-the-counter and prescription medications, so if you are breastfeeding you should check the ingredients of the products you use for treatment of skin conditions.
According to Healthwise, a publication of the University of California, San Diego, salicylic acid topical treatments may pass into the breast milk of nursing mothers, and thus get ingested by the baby. As this medication is not recommended for children under two years of age, it is important to tell your doctor you are breastfeeding so you can make an informed decision about whether this is the right treatment plan for you.
Rather than using a medication that might potentially be harmful to your child, you can ask your doctor whether there are any alternative treatments that are known to be safe for women who are breastfeeding. You should also consider whether you really need the condition to be treated. For example, acne is not pretty or pleasant, but you don't necessarily need to use medication for it.
If you must take salicylic acid and don't want to risk your baby ingesting any of this medication, you can pump your milk and toss it out during the treatment period and bottle feed your baby with either previously pumped milk or formula. This would keep you from having to stop breastfeeding for good while still allowing you to get the treatment you need.
According to the Mayo Clinic, you should speak with your doctor so that you can weigh the potential benefits against the potential risks of using salicylic acid when breastfeeding. No studies have been done to determine its safety for nursing babies.