Thanks for the valuable info you had given.I've taken a close look at your question & the reply and come up with the following answer:
Long term use of Diphenhydramine on a regular basis may cause side effects.
It can be taken occasionally or for short duration for relief from symptoms of allergy.
Individuals who take Benadryl for extended periods of time may experience low blood pressure and heart palpitations
It may also have addictive effects.
Do not use antihistamines like Diphenhydramine for more than approximately ten days or more than two or three times a year without consulting a doctor.
The underlying cause of allergy should be detected & then treatment should be done accordingly.
So I advice you to take this medication only as prescribed by your doctor & for short duration.
Hope this helped
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