Fentanyl is quite potent, it is 100x more potent than morphine as a mu-opioid receptor agonist. Thus the doses in pharmaceutical preparations are small -- a Duragesic patch may deliver 25 to 100 micrograms of fentanyl (0.025 to 0.1 milligrams)per hour when worn.
It has the same effects as other opioids: analgesia (pain relief), euphoria, sedation (but not sloppy sedation like with alcohol or benzodiazepines, there is little motor impairment except with very high doses and one retains most of their social inhibition if they wish to not talk ... however...), increased DESIRE to talk to other people (is another effect of opioids caused by the surge of dopamine released in the nucleus accumbens), itching, anxiolysis (relief from anxiety), and occasionally hyperproductivity and often enthusiasm / motivation (despite the sedative effect) due to the dopaminergic actions of opioids.
Fentanyl, like other opioids, is one of the most intrinsically euphoric drugs (opioids and psychomotor stimulants are the most intrinsically / non-set and setting dependent euphoric drugs).
Fentanyl can be highly psychologically and physically addictive, like other opioids.
In its prescription form, fentanyl is known as Actiq, Duragesic, and Sublimaze.
When prescribed by a physician, fentanyl is often administered via injection, transdermal patch, or in lozenge form. However, the type of fentanyl associated with recent overdoses was produced in clandestine laboratories and mixed with (or substituted for) heroin in a powder form.
Mixing fentanyl with street-sold heroin or cocaine markedly amplifies their potency and potential dangers. Effects include: euphoria, drowsiness/respiratory depression and arrest, nausea, confusion, constipation, sedation, unconsciousness, coma, tolerance, and addiction.