jeckrn, BSN, RN 1,868 Posts Specializes in EMT, ER, Homehealth, OR. Has 17 years experience. Sep 12, 2012 It is not a first-choice, first-line med but it exists and sometimes used.The problem is, benadryl works really slow comparing with tempo of anaphylaxis. The start of action is expected, under normal circumstances, within 15-20 min., and the patient can die well before this time passes. Also, benadryl, especially given IV, can cause hypotension, doesn't directly treat bronchospasm and its sedative effect may not be desirable. I carry benadryl SL pills and SQ auto-injector (the latter thing isn't available in the USA so I buy it in Europe) in my "allergy pack" with total dose 200 mg. I use them only for local and less severe reactions. Paramedics and ER personnel usually inject it later, IV or SQ, but only after blood pressure is under good control.I worked the ED for years and gave more Benadryl for allergic rx's then epi.