No. Both products contain an antihistamine. This is a
therapeutic duplication which may result in significant additive
side effects.
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My friend had taken 25 mg of Diphenhydramine, and 500mg of
Methocarbamol, and didn't have any problems. They reported being
real sleepy. Ask your Doctor, though. What works for others, may
not work for you.
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Since Theraflu contains phenylephrine it says you should not
take it with Benadryl because it contains acetaminophen and
diphenhydramine and can cause a reaction.