Here is great source for drugs to avoid during pregnancy and nursing.
My philosophy is to avoid all drugs if I can during the 62 days of pregnancy. I try and vaccinate if necessary prior to breeding. Also I try to deworm prior to breeding but, as we all know, stuff happens..
There is also the risk of drugs passing into the milk and affecting puppies while they are being nursed.
This page, by Dr. Holly Nash, includes all sort of drugs from dewormers to salves to antiobiotics. Note that there is a column for “Safety Studies in Dog not Available” . Be sure to check that column when checking on the safety of a product.
I have found that this information contradicts some of the advise given by my local vets especially when it comes to dewormers.
http://www.peteducation.com/article.cfm?cls=0&cat=1303&articleid=856