Fentanyl is a narcotic analgesic with a potency at least 80 times that of morphine. Fentanyl and its derivatives (Alfentanil, Sufentanil, Remifentanil and Carfentanil) are used as anaesthetics and analgesics in both human and veterinary medicine (Carfentanil). They are subject to international control as are a range of highly potent non-pharmaceutical fentanyl (NPF) derivatives, such as 3-methylfentanyl, synthesised illicitly and sold as ‘synthetic heroin’, or mixed with heroin.
Different fentanyl derivatives have been developed by the legitimate pharmaceutical industry by adding various substituents to the basic molecule in order to modify potency (Table 1). This approach has been mimicked by chemists in clandestine laboratories to produce illicit NPF derivatives. Depending on the position of the substituents, some of the resulting molecules may exist as enantiomers e.g. the isomers of 3-methylfentanyl, which have differing analgesic potencies depending on which enantiomer is used. Van Bever et al. (1974) reported that the introduction of a methyl group (-CH3) into the 3 position of the piperidine ring, increases analgesic potency. The trans isomer was slightly more active than fentanyl, but its corresponding cis form was eight times more active. They found that activity of the cis isomer resided in one enantiomer, namely cis(+) 3-methylfentanyl which was16 times more potent than fentanyl, whereas the cis(-) form was 120 times less potent (see non-pharmaceutical fentanyls below).
3-Methylfentanyl (3-MF, mefentanyl) is an opioid analgesic that is an analogue of fentanyl. 3-Methylfentanyl is one of the most potent drugs that has been widely sold on the black market, estimated to be between 400-6000 times stronger than morphine  depending on which isomer is used (with cis isomer being the more potent one).
While 3-methylfentanyl was initially sold on the black market for only a short time between 1984-1985, its high potency made it an attractive target to clandestine drug producers, as racemic 3-MF is 10-15x more potent than fentanyl and so correspondingly larger amounts of cut product for street sales can be produced for an equivalent amount of effort as for producing fentanyl itself; one gram of 3-methylfentanyl might be sufficient to produce several thousand dosage units once diluted for sale. 3-MF has thus reappeared several times, at various places around the world, and is currently a problematic drug of abuse in Scandinavian and Eastern European countries such as Finland  and Estonia 
Other opioid analogues even more potent still than 3-MF are known, such as carfentanil and ohmefentanyl, but these are significantly more difficult to manufacture than 3-methylfentanyl and have not been so well accepted as street drugs.
3-Methylfentanyl has similar effects to fentanyl, but is far more potent due to increased binding affinity to its target site. Since fentanyl itself is already highly potent, 3-methylfentanyl is extremely dangerous when used recreationally, and has resulted in many deaths among opiate addicts using the drug. Side effects of fentanyl analogues are similar to those of fentanyl itself, which include itching, nausea and potentially serious respiratory depression which can be life-threatening.