I believe

I went to a conference on Friday.

One of the talks was on the high percentage of marijuana and heroin use by members of the armed forces while serving in Vietnam.

This article from 2002 is long but does a fairly good job of reviewing many of the prevailing conditions at the time:

http://www.library.vanderbilt.edu/central/Brush/American-drug-use-vietnam.htm

I happened to see a Vietnam veteran today.

In the midst of taking his history I asked him about the drug culture during his time in Southeast Asia.

He remembers what he was told by a member of his unit when he first arrived in Vietnam:

"Man made booze, God made grass; I believe in God. Do you believe in God?"

He went on to say, "I was 18, scared to death, and looked up to those guys. I started smoking marijuana soon after I arrived."