MacLaine admits to trying pot brownies provided by VIP Robert Mitchum (with whom she had an affair) in one of her books, where she also says she tried smoking pot in a London hotel room.
Two Mules was filmed in Mexico and written by director Budd Boetticher, who lived there. According to Wikipedia, Boetticher had planned on using Mitchum as the male star of the film, and the part of Sister Sara was originally offered to VIP Elizabeth Taylor.
Directing the movie was Don Siegel, who directed the 1949 movie The Big Steal, the first film Mitchum made after his bust for marijuana. It was also filmed in Mexico, where co-star Jane Greer reported locals were always trying to foist joints on Mitchum.
The Two Mules film score was composed by Ennio Morricone, who's named as a pot smoker on the web, although I have not found confirmation. Since he's the brilliant composer of "The Good, The Bad, and the Ugly" as well as "The Mission" and other masterpieces, it would be nice to know!
In 1960, Boetticher departed from the Western genre to make The Rise and Fall of Legs Diamond, a film loosely based on the lives of Jack Diamond and Arnold Rothstein, the US's first major drug dealer (who appears in F. Scott Fitzgerald's The Great Gatsby as well as Damon Runyan stories). Boetticher had a role in the 1988 Robert Towne movie Tequila Sunrise, starring Mel Gibson as a man who has a connection with a Mexican dealer (Raul Julia).
MacLaine's now playing the American mother going toe to toe in Downton Abbey with matriarch Maggie Smith (who starred in Travels with My Aunt, based on Graham Greene's story about an eccentric woman with a marijuana connection).
Also recently seen: Bunny O'Hare (1971), starring Bette Davis as a widow who heads to Mexico on the back of a motorcycle driven by Ernest Borgnine as the two pose as hippies to pull off a string of bank robberies. Ernest puffs in the movie and Bette refuses when offered, but asks some intelligent questions about it. She gets the last word, and it's a doozy.