California's implied consent law requires a driver arrested on suspicion of DUI to submit to either a breath or blood test.  If the person refuses, the officer can request a warrant for a blood draw.  An officer cannot force a blood draw.  They need to get a warrant.  This law was established in 2013 by the United States Supreme Court in the case titled Missouri v. McNeely.  In that case, the Court ruled that law enforcement officers could not conduct a warrantless blood draw from a driver who did not provide consent and if there was not an applicable exception to the warrant requirement.  The Court stated that a blood draw is a "search" under the 4th Amendment of the United States Constitution and, therefore, officers need either a warrant or an exception to the warrant requirement, such as consent, in order to perform a blood draw.  The Court also overruled the previous law which allowed officers to conduct a forced blood draw based on the theory that the natural elimination of alcohol would be considered an "exigent circumstance," which is an exception to the warrant requirement.  The McNeely Court stated instead "that while the natural dissipation of alcohol in the blood may support a finding of exigency in a specific case, it does not do so categorically.  Whether a warrantless blood test of a drunk-driving suspect is reasonable must be determined case by case based on the totality of the circumstances."

The Supreme Court decided another case regarding DUI's and warrants, Mitchell v. Wisconsin.  This decision makes an exception of sorts to the rule that it established in the McNeely case.  In the Mitchell decision, the Court stated "When police have probable cause to believe a person has committed a drunk-driving offense and the driver's unconsciousness or stupor requires him to be taken to the hospital or similar facility before police have a reasonable opportunity to administer a standard evidentiary breath test, they may almost always order a warrantless blood test to measure the driver's BAC without offending the Fourth Amendment."  The Court explained that this situation would fall under the "exigent circumstances" exception to the warrant requirement.  The Court reasoned that "a driver so drunk as to lose consciousness is quite likely to crash, giving officers a slew of urgent tasks beyond that of securing medical care for the suspect—tasks that would require them to put off applying for a warrant. The time needed to secure a warrant may have shrunk over the years, but it has not disappeared; and forcing police to put off other urgent tasks for even a relatively short period of time may have terrible collateral costs." 

The law as it stands now still requires officers to get a warrant for a blood draw.  But the Mitchell decision essentially carves out a new exception to this requirement: if the driver is taken to the hospital and is unconscious and/or in a "stupor" then officers are not required to get a warrant.  This is considered an "exigent circumstance" and they can go ahead and have the blood drawn without first obtaining a warrant.