I just took a mucinex. will i be okay if i take benadryl (diphenhydramine) as well?

Not a good idea: You have to know Mucinex help thin the phlegm and Benadryl (diphenhydramine) has drying effect on mucous or phlegm, so you are defeating the purpose of taking musinex if you need to take antihistamine with Mucinex for allergy, i will recommend takind over the counter Claritin which may not be as drying as benadryl, (diphenhydramine) and dont forget to drink lot of fluids.