Sen. Feinstein has been a long-time foe of legal marijuana.  As Mayor of San Francisco, she sabotaged implementation of a voter-approved initiative not to prosecute marijuana crimes.  She has prided herself as a staunch supporter of federal narcotics enforcement and co-chairs the Senate Caucus on International Narcotics Control, which has taken a hard line on drug enforcement.

Sen. Feinstein has never endorsed marijuana decriminalization, despite California's 40 years of success with it.  In the Senate, she co-sponsored legislation to penalize drug offenders with loss of food stamps and other federal benefits.

Sen. Feinstein actively opposed California's medical marijuana initiative Prop 215.     Since then she consistently sought to limit the scope of protection for medical marijuana  and consistently  opposed efforts to protect California's law from federal interference.  Only at the insistence of veterans' groups did she finally cast a vote  in favor of allowing access to medical marijuana by VA patients.    Only this year, after being challenged by Sen. De Leon,  did she finally shift her position by declaring support for the STATES act, which would protect state law from federal interference.  However, she has declined to support more sweeping legislation that would federally reschedule and legalize medical marijuana.

S.1276 - Cannabidiol Research Expansion Act (2017-18)

S. 3269 Cannabidiol Research Expansion Act (2015-16)

Sen. Feinstein has supported only the weakest bills to expand marijuana research with CBD, as an alternative to stronger bills that would expand research with THC.

"I think marijuana has potential dangers to it. I think they need to be looked at - calibrated. I think we need to be concerned about young people, without judgement, particularly in cars. Particularly on Saturday night, smoking marijuana, candidly."
1/1/2017
http://www.rollingstone.com/politics/features/sen-cory-booker-is-trying-to-end-federal-weed-prohibition-w495343

"There may very well be a place for medical marijuana," California's senior senator said in a statement before the vote. But "to take a federal position on this before the research is done ... is putting the cart before the horse."

allowing states to enforce their own laws without federal interference would probably discourage research, "because you won't be able to go against a state that has implemented (its marijuana laws) and has its own rules." And federal intervention is appropriate, Feinstein said, unless a state has "a strong regulatory system in place."

Feinstein was a vocal opponent of the 2010 initiative, which she called "a jumbled legal nightmare that will make our highways, our workplaces and our communities less safe." She also opposed the 1996 ballot measure that legalized medical marijuana.

She told an interviewer last year that as a member of the state parole board for women in the 1960s, she saw many convicts who "began with marijuana and went on to hard drugs." Similarly, at Thursday's hearing, Feinstein described marijuana as a "gateway drug" with a "growing addiction factor," a view the drug's advocates say is unsupported by research.

"Federal law enforcement agents should not arrest Californians who are adhering to California law," said Feinstein, who is facing a challenge in the Democratic primary from state Sen. Kevin de Le�n. 5/1/18 https://www.washingtonpost.com/news/powerpost/wp/2018/05/01/sen-dianne-feinstein-says-she-no-longer-opposes-legal-marijuana/?noredirect=on&utm_term=.50666231dd76