Banadryl consists of diphenhydramine and it tend to increase heart rate. So there is no serious risk of decrease in heart rate with Banadryl. Melatonin is comparatively safe drug still it may decrease BP to some extent.Over all Benadryl will not decrease your heart rate or pulse. You can take it.anadryl consists of diphenhydramine and it tend to increase heart rate. So there is no serious risk of decrease in heart rate with Banadryl. Melatonin is comparatively safe drug still it may decrease BP to some extent.