Acetyl fentanyl is a relative of fentanyl and is five times more potent than heroin as a painkiller. The illegally produced compound may be secretly mixed with heroin to make it a more potent product, or may be sold in pills disguised as oxycodone. Users themselves may not be aware that they are ingesting it.

Excerpt from Live Science:
https://www.livescience.com/47451-emerging-drug-acetyl-fentanyl.html

Acetylfentanyl is a synthetic opioid that was originally described in the 1960s. It's less potent than fentanyl and more potent than morphine. The drug has contributed to many fatalities, often due to it being mixed with heroin or misrepresented as heroin. In terms of effects, it appears similar to fentanyl.

The Drug Classroom:
https://thedrugclassroom.com/video/acetylfentanyl/

Fentanyl has very euphoric properties, and is a favorite of drug abusers. Medical personnel abuse fentanyl in the parenteral form, but most street use comes from substituting fentanyl for heroin by drug dealers or from the illicit extraction of fentanyl from used or new transdermal patches. Recently, a close relative to medical fentanyl, acetyl fentanyl, has surfaced as a drug of abuse that has potentially fatal consequences.

Excerpt from Emergency Medical News:
https://journals.lww.com/em-news/fulltext/2013/12000/InFocus__Acetyl_Fentanyl_New_Drug_of_Abuse_More.4.aspx

Acetylfentanyl

Duration: Acetyl-Fentanyl is an opioid analgesic substance that is an analogue of Fentanyl. It's potency is roughly 6.67 times that of Morphine. Making it about 15 times less potent than its parent compound Fentanyl.
Route	Onset	Duration	After Effects
Tripsit Factsheets
Acetylfentanyl Basic Information: http://drugs.tripsit.me/acetylfentanyl
All ROAs:	5-20 minutes	1-4 hours	1-8 hours
Aliases: acetyl-fentanyl a-f
Effects: Euphoria, Dry Mouth, Mood lift, Itchiness, Relaxant, Constipation, Pupil constriction, Analgesia.

World Health Organization 2015:
Acetylfentanyl is clearly being used for non-medical purposes,although the incidence and prevalence of its abuse cannot be accurately estimated because, in part, it is not routinely tested for in forensic toxicology. It has been identified in confiscated material being trafficked illicitly in the United States, Europe and Japan. It is sold over the Internet where it is sometimes promoted as a "research chemical", and its use is discussed on drug-user websites. Acetylfentanyl has been associated with at least 52 deaths in the United States (where it has been emergency scheduled under the Controlled Substances Act) and with deaths in Europe as well.

Users have reported using acetylfentanyl via insufflation, smoking, and through the intravenous routes of administration. Actual doses are thought to be in the microgram range. Effects reported by users appear indicative of heroin/fentanyl-like effects.

Acetylfentanyl is being controlled as an abused substance in some member states. Its use as an abused substance is unequivocal, and has resulted in several deaths. Precise estimates of the incidence and prevalence of acetylfentanyl's abuse, however, are difficult at present

WHO (PDF 17 pages):
https://www.who.int/medicines/access/controlled-substances/5.2_Acetylfentanyl_CRev.pdf

DEA (PDF 1 pages):
https://www.deadiversion.usdoj.gov/drug_chem_info/acetylfentanyl.pdf

Acetylfentanyl is a synthetic opioid. It is closely related to fentanyl, which is controlled under the United Nations Single Convention on Narcotic Drugs of 1961. Data suggests that acetylfentanyl is likely to be a potent opioid narcotic analgesic and may have an abuse liability and dependence potential in humans; these effects may be broadly comparable to fentanyl.Acetylfentanyl has been available in the European Union since at least 2013 and has been detected in 8 Member States and Norway. In most cases it has been seized as a powder, but other forms such as liquids and tablets have also been detected. The detected quantities are relatively small; however, there is still a cause for concern given the high potency of the substance. It appears that acetylfentanyl usually originates from chemical companies based China. There is no indication of illicit production sites within the European Union.

European Monitoring Centre for Drugs and Drug Addiction Acetylfentanyl (PDF 21 pages):
http://www.emcdda.europa.eu/system/files/publications/2693/TDAS16001ENN.PDF

Erowid Experience Vault:
https://erowid.org/experiences/subs/exp_Acetylfentanyl.shtml

• An opioid analgesic
• An analog of fentanyl
• Estimated to be fifteen times more potent than morphine
• It has never been licensed for medical use
• It has been sold as a designer drug

Acetylfentanyl was discovered at the same time as fentanyl itself and had only rarely been encountered on the illicit market in the late 1980s. However, in 2013, Canadian police seized 3 kilograms of acetylfentanyl

Acetylfentanyl overdosage has been reported to closely resemble heroin overdosage clinically.

DEA informed about 52 confirmed fatalities involving acetylfentanyl in the United States between 2013 and 2015.