If the character isn't in a practicing medical profession or already IV addict, its implausible. Putting a needle INTO a vein needs practice and some spite. Besides that, you'd need to know about the right dose. Most Heroin addicts report they noticed when they nearly died, so you may be indifferent or powerless but nice may be something else.
When people get medically assisted suicide in Europe (terminally ill), before the death inducing opiates are injected, the patient is properly sedated with benzodiazepines to prevent stress or anxienty. These meidcations do not let you pass out, but here we call them the "I don't give a fuck"-Pills. That Sure is a "Nice" way to go.
Once had a wisdom tooth operation with benzos. I winked at the cute nurse while she pried with a metal bar inside my jawbone.
