When I queried a drug with a pharmacist she said something I didn't perfectly understand about how it was metabolised and how big the resulting molecules were, as though big enough ones don't cross the blood/milk barrier.
It may therefore be the case that some drugs will definitely pass into milk by the nature of their chemical composition, and some not, before you even consider the effect of said drug on the breastfed child.
I'm sorry I haven't a better answer. I am not a biochemist! The links provided earlier are great.
I had the same problem as you but I already knew that the compatible drugs wouldn't have touched my arthritis and the only ones that have worked so far are a total no no when bf.
I made the decision not to stop bf so that I could take the drugs and although this wasn't prehaps my wisest idea, especially at first, we are still going strong at 11 months.
I went to see my GP because my consultant and nurse weren't supportive of my decision to continue bf and asked him to refer me to a physio as another doctor at the hospital had suggested this as an alternative.
I have been receiving accupuncture (from a very supportive physio) and am able to cope with the pain/stiffness because of this and plan to carry on bf as long as I possibly can.
So, finally, my point is if the drugs don't work there are other short term options to consider although you may have to fight for them as I found because they probably won't be offered to you.
There's not a full range of data for every drug and you are right that some drugs are OK with older babies and more of a concern with younger babies. It depends on the effects on the infant and the half life. Some drugs affect lactation so are not recommended at any age.
LactMed is a really good resource for information, there's also a free phone app if you want to take the information with you to the hospital.
The BFN have a drugs in breastmilk helpline that might also be helpful or you can email them here.
Lol at currently 9m old DS2 being considered extended!
Having now seen the consultant for my arthritis, they are putting me on DMARDs, one of three types - One is safe for BFing, one unknown, and one unsafe.
I understand the unethicalness of testing the babies if its known to pass into BM, and am glad the first one has already been found to be compatible so I won't need to stop yet (and hopefully making my whole post irrelevant, unless that drug doesnt work for me...) I have read the findings of the study for the first drug, and it was tested on small EBF babies.
Which leads to my query/irritation. If effects of drugs in BM are tested on EBF babies, is it really correct to say that a drug is completely incompatible with breastfeeding a one year old who is possibly only on one or two feeds a day? Two year old? Three? Surely the age and weight of the child, plus a low actual amount of milk makes even an unsafe drug safer than an EBF newborn?
Just in case it needs saying, obviously I won't put my child at risk by carrying on feeding if I have to go onto one of the other drugs, I'd just rather not stop yet unless I actually have to.