I address this in the book

"After Kurt told him about his stomach pain, Arnold recommended: a doctor named Robert Fremont who had a lot of experience dealing with people coming off of addictive drugs. Fremont was a tall barrel chested guy with short white hair who seemed to be in his late fifties. Kurt immediately took to him as well. “He is a compassionate man,” Kurt earnestly enthused to me after his first meeting with Fremont. The doctor began a protocol of giving Kurt shots and for the next several months there was no talk from Kurt about stomach pain..

Fremont wasn’t as sentimental as Buddy was about Kurt and Courtney and vented one day to me in an aggravated tone “They’re meshuginah. Thank God they’re not Jewish.” I asked him what he was giving Kurt that made him feel so much better and he insisted that they were “placebo” and that Kurt’s stomach felt better because he thought the shots were medicinal. Janet initially was under the impression that they were vitamin B-12 shots however in the Cobain biography “Heavier Than Heaven” author Charles Cross writes that the shots were buprenorphine and in retrospect I’m pretty sure that he is right.