I'm not familiar with thera-flu, but doesn't that have a antihistamine in it? If so, you wouldn't want to take extra antihistamine. If thera-flu doesn't have a antihistamine, it would be fine to...
Since Theraflu contains phenylephrine it says you should not take it with Benadryl because it contains acetaminophen and diphenhydramine and can cause a reaction.
http://www.mercksource.com/pp/us/cns/cns_hl_pdr.jspzQzpgzEzzSzppdocszSzuszSzcnszSzcontentzSzpdrotczSzotc_fullzSzdrugszSzfgotc331zPzhtm
I just ran into this same question myself, and I believe your...