There's something similar in recovery, where it's "treat the addiction that's most likely to kill the addict." So if someone was shooting up, also abusing alcohol and smoking, you'd want to go after the IV drug use first. A lot of people feel bad if they stop abusing one drug but they're still not completely clean, but I think the harm reduction model (which I greatly approve of even if it didn't work for me) has made some progress there.