According to the Drug Interactions Checker at Drugs.com these two drugs have a 'moderate' interaction between each other. This is because these two drugs have similarities which can make each 'stronger'.
Here's the warnning provided by that site.
Using diphenhydrAMINE together with cyclobenzaprine may increase side effects such as drowsiness, blurred vision, dry mouth, heat intolerance, flushing, decreased sweating, difficulty urinating, abdominal cramping, constipation, rapid heart beats, confusion, and memory problems. Side effects may be more likely to occur in the elderly or those with a debilitating condition. You should avoid or limit the use of alcohol while being treated with these medications. Also avoid activities requiring mental alertness such as driving or operating hazardous machinery until you know how the medications affect you. It is important to tell your doctor about all other medications you use, including vitamins and herbs. Do not stop using any medications without first talking to your doctor.
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