A 4:3 naltrexone: 5-methyl-2-furaldehyde cocrystal and its use as an opioid antagonist are disclosed. The invention also relates to a drug-in-adhesive transdermal patch containing the analgesic fentanyl, a mu opioid agonist, or an analog thereof and a 4:3 naltrexone: 5-methyl-2-furaldehyde cocrystal, as an opioid antagonist. A transdermal patch of the invention is tamper-resistant and an abuse deterrent which protects against drug misuse or abuse. The invention also provides a method of treating pain, such as acute, chronic, or intermittent pain, by applying a drug-in-adhesive transdermal patch according to the invention to the skin of a patient in need thereof. Also disclosed is an improved transdermal patch for administering fentanyl or an analog thereof, or for administering a mu opioid agonist, the improvement wherein the transdermal patch contains a 4:3 naltrexone: 5-methyl-2-furaldehyde cocrystal in an abuse limiting amount. The improved transdermal patch may be a drug-in-adhesive transdermal patch or a reservoir transdermal patch.