3 If a patient on fentanyl (transdermal) is to be transitioned to another opioid, the morphine equivalent is: Fentanyl (transdermal) 25 mcg/ hr = 45 mg morphine (oral)
If a patient is on morphine (oral), the fentanyl (transdermal) equivalent is: Morphine (oral) 90 mg = 25 mcg /hr fentanyl (transdermal)
Equivalencies used are different because the authors recognize that conversion may not be bi-directionally equivalent, and therefore, adjustments have been made so that conversions to or from fentanyl remain conservative.
Fentanyl analogues may be hundreds of times more potent than street heroin, and tend to produce significantly more respiratory depression , making it much more dangerous than heroin to users. Fentanyl is used orally, smoked, snorted, or injected. Fentanyl is sometimes sold as heroin or oxycodone , often leading to overdoses. Many fentanyl overdoses are initially classified as heroin overdoses.  The recreational use is not particularly widespread in the EU with the exception of Tallinn, Estonia, where it has largely replaced heroin. Estonia has the highest rate of 3-methylfentanyl overdose deaths in the EU, due to its high rate of recreational use. 
Opioids are a class of compounds that elicit analgesic (pain killing) effects in humans and animals by binding to the µ-opioid receptor within the central nervous system . The following table lists opioid and non-opioid analgesic drugs and their relative potencies . Values for the potencies represent opioids taken orally unless another route of administration is provided. As such, their bioavailabilities differ, and they may be more potent when taken intravenously . Methadone is different from most opioids considering its potency can vary depending on how long it is taken. Acute use, 1–3 days, yields a potency about × stronger than that of morphine and chronic use (7 days+) yields a potency about to 5× that of morphine due to methadone being stored in fat tissue, thus giving higher serum levels with longer use. [ citation needed ] Similarly, the effect of tramadol increases after consecutive dosing due to the accumulation of its active metabolite and an increase of the oral bioavailability in chronic use; this effect becomes less significant again with even longer use as tolerance develops. [ citation needed ]