Hi- 
Catherine was correct that Differin is a topical medication that can be used w/acne. If she has a high co-pay because it is on higher tier, Retin A is an old stand-by that was used before Differin and so would be cheaper. I would recommend getting ahold of Hale's book and giving the info to mom so she can make an informed decision. It is interesting to note that Doxycycline is an L3 for acute use and L4 for chronic use. He states (this is in the 11th edition as I don't have the latest edition at home) "short term use (several weeks) is not contraindicated. No harmful effects have yet been reported in breastfeeding infants but prolonged use is not advised". 
Usually, when you have inflammatory acne, you need to get it under control w/an antibiotic (po) and then can switch to topical only. Azelex is a topical antibiotic that might work better than the erythromycin that she is using. A "standard" in treating acne is to use topical retinoid, a topical antibiotic and add in po antibiotic if lesions are inflammatory. From personal experience, I would take the po until things calmed down, and then keep only the topicals. Eventually I could drop the topical antibiotic and now am only on Differin. (it is really lousy to be 42 years old and still dealing with acne!!) So, to sum it up, using topical retinoid and topical anitbiotic and adding in the doxycycline for just a few weeks might be an option. (Of course-this is for information purposes only and not a formal recommendation for treatment!)  Again, mom should be given all the info so she can make an informed choice as to the risks vs the benefits.
Warmly,
Stacy D. Kucharczk, MSN, RN, CPNP, IBCLC
BestFeeding Lactation Services, LLC

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