In a phone interview with the Associated Press, Feinstein said one of her concerns is what she called the "very substantial" risk of pot-impaired drivers on the roads.
Feinstein says she hopes California chooses not to follow Colorado and Washington in approving the sale of marijuana for recreational use.
Several legalization petitions are circulating in California this year, although none has yet qualified for the ballot.
California became the first state to legalize the use of marijuana for medicinal purposes in 1996, but voters rejected a 2010 ballot initiative seeking to legalize it for recreational purposes.