Gov. Arnold Schwarzenegger vetoed a bill on Tuesday that would have eased rules on how much medical marijuana patients could possess in California.
State voters approved a measure in 1996 to allow medical marijuana, but the initiative has been subject to a continuing battle pitting it against federal rules.
The bill that Schwarzenegger vetoed would have removed quantity limits on the drug for California patients.
"Reasonable and established quantity guidelines allow medicinal marijuana patients to seek relief from symptoms free from legal questions and permit law enforcement to carry out the law," Schwarzenegger wrote in a note to the State Senate explaining his veto.
"Enactment of this bill would create uncertainty in this area of the law thereby making it more difficult for law enforcement to determine when a person was in possession of marijuana for medicinal purposes."
Schwarzenegger has previously spoken out in favor of medical marijuana.