Brief Answer:
Yes you can take but under supervision of your doctor

Detailed Answer:
Hello onomaile

This is Dr Rahul Rathee and I am glad to answer your query----
(1) I am sure that your doctor must have told you, metoprolol has two main actions, that is, to reduce your heart rate and reduce your blood pressure
(2) Benadryl has an additive effect on metoprolol meaning by that it increases the effects of metoprolol
(3) So the dose of metoprolol which you are presently taking , its effect will be increased when you start benadryl and there will be greater reduction of heart rate and blood pressure
(4) You can take metoprolol and benadryl together but under supervision of your doctor because he might consider reducing the present dose of metoprolol to counteract the additive effects of benadryl

I hope I have answered your doubt to your satisfaction but if you still have any query regarding this , Please feel free to ask me
Regards
Dr Rahul

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