A study of Vietnam War veterans found that 20-25% were addicted to heroin, but upon returning to the US, nearly all provided clean urine tests, and 95% remained clean, suggesting addiction is not solely a brain disease that obliterates choice.

“so the Vietnam War you had about 20 to 25% of soldiers US soldiers who were in Vietnam were addicted to heroin they're regular frequent users of heroin and President Nixon he decides I don't want to let all these heroin addicts back into the United States it was a significant number of people so he decides to start a program that is called operation golden flow and the deal is this if you want to come back into to the United States as a soldier who's been in Vietnam you have to produce a clean urine test if you do not produce a clean urine test you will be left in Vietnam so if addiction basically uh obliterates the capacity to make any choices any decisions you would expect that 25% of soldiers in Vietnam would be left in Vietnam now what actually happened is that nearly every single one provided clean urine and when they got back into the United States the vast majority of them about 95% managed to stay clean the 5% that relapsed they tended to be people who had used drugs before the war”