The Breastfeeding Network have produced a range of factsheets, which are maintained by their Drugs in Breastmilk information service team of pharmacists. 

Drugs in breastmilk fact sheets.

If you are unable to find the information you are looking for you can contact the team by druginformation@breastfeedingnetwork.org.uk  Provide as much information as possible (including age of baby, name of medication or drug, dose and frequency).