There seems to be an issue woth ohdsi.org's ATLAS install, so I can't link you a ConceptSet expression, but I am able to create cohorts, so I will create a concept set inside that for your review:
I created the cohort, and then added a new concept set to the cohort.
I did a vocabulary search for 'phenytoin'. I located the row that says the class is 'Ingredient', a Standard concept from RxNorm. I clicked the shopping cart to add it to my concept set.
Then i went back to my concept set view, and clicked the 'descendents' checkbox so that all the drugs found under phenytoin will be used.
Here's the cohort definition: http://www.ohdsi.org/web/atlas/#/cohortdefinition/1729468
You can go into the Concept Sets tab, chose the concept set 'phenytoin', and then look at the included concepts. You will see you will find all the branded and clinical drug forms that are under the ingredient (there are over 1000!). This is a much better approach to selecting your drugs of interest than manually clicking on 1000 items in a text search.