"When wearing a Duragesic patch, check with your doctor before taking any other central nervous system (CNS) depressant drugs. The combined effect can lead to difficulty breathing, hypotension, and possibly coma.
Drugs in this category include the following:
*Sleep-inducing antihistamines such as Benadryl and Phenergan"
source: http://www.drugabusehelp.com/drugs/fentanyl/
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