The objective of this study is to compare the rate of absorption and bioavailability of fentanyl sublingual spray 400 mcg to fentanyl citrate 100 mcg by intramuscular injection.

Inclusion Criteria:

• Meets protocol-specified criteria for qualification and contraception
• Good access to veins on both sides
• Willing and able to remain confined in the study unit for the entire duration of each treatment period and comply with restrictions related food, drink and medications
• Voluntarily consents to participate and provides written informed consent prior to any protocol-specific procedures

Exclusion Criteria:

• Intolerance to venipuncture or injections
• Presence or history of oral disease, irritation or piercings
• Allergy or adverse response to fentanyl, naltrexone, or related drugs
• Tattoos, scarring, or other skin abnormality at planned injection sites
• History or current use of over-the-counter medications, dietary supplements, or drugs (including nicotine and alcohol) outside protocol-specified parameters
• Signs, symptoms or history of any condition that, per protocol or in the opinion of the investigator, might compromise: 1) the safety or well-being of the participant or study staff; 2) the safety or well-being of the participant's offspring (such as through pregnancy or breast-feeding); 3) the analysis of results