Can I use Fentora if I have a history of a slow heartbeat?

A history of a slow heartbeat doesn't necessarily mean you can't use Fentora (fentanyl), a morphine-like drug for breakthrough cancer pain. But Fentora might worsen your condition, placing you at higher risk for one of this drug's most serious side effects, a potentially fatal slowdown of breathing. Talk to your doctor so the two of you can weigh the risks of using the drug versus your need for greater pain relief.