Federal charges against Derby nurse Heather Alfonso center on a powerful and addictive painkiller called Subsys, which has been heavily marketed by the Arizona-based manufacturer Insys Therapeutics, federal officials confirmed Wednesday. Alfonso, 42, of Middlebury, pleaded guilty Tuesday in U.S. District Court in Hartford to receiving $83,000 in kickbacks from January 2013 until March 2015 from a pharmaceutical company that makes a drug used to treat cancer pain. In pleading guilty, Alfonso admitted that the money she was paid influenced her prescribing of the drug, according to the U.S. Attorney’s Office for Connecticut, which is prosecuting the case. Receiving kickbacks in exchange for billing charges to a federal health care program is illegal. While the company and drug are not named in the indictment, a prosecutor revealed in court Tuesday that the case involves Subsys and Insys Therapeutics.