In February, the FDA approved changes to the warning label for the Duragesic (Fentanyl) pain patch, including adding warning patients using the patch not to drink grapefruit juice or expose the patch to a direct heat source.

Dose increases should be approached with caution, and patients should avoid grapefruit or grapefruit juice… (Emphasis added.)

. . . .

Patients and caregivers should also be advised to avoid exposure of the patch application site to direct external heat sources because of the potential for temperature-dependent increases in fentanyl release that may result in overdose and death. (Emphasis added.)

Heat sources include heating pads, electric blankets, heat lamps, saunas, hot tubs, and heated water beds; sunbathing and hot baths should likewise be avoided. Patients in whom fever or increased core body temperature from strenuous exertion develops should be monitored for opioid adverse events, and fentanyl dosage should be adjusted accordingly.

Source: FDA Safety Changes: Ultram ER, Reyataz, Duragesic