At least in California, the answer is yes (State supreme courts in Oregon and Montana ruled the same way)
In Gary Ross vs. Raging Wire Telecommunications Inc., the California Supreme Court upheld the firing of Ross after he failed the pre-employment drug test required of all new employees -- even though his physician had recommended he use marijuana to treat chronic pain.
In agreeing with a lower-court decision, the state's high court ruled, in a 5-2 decision written by Associate Justice Kathryn Mickle Werdegar (http://www.hreonline.com/pdfs/08012010Extra_RossOpinion.pdf) , that "Nothing in the text or history of the Compassionate Use Act suggests the voters intended the measure to address the respective rights and duties of employers and employees."
It noted that under California law, "an employer may require pre-employment drug tests and take illegal drug use into consideration in making employment decisions."
In the dissent (it starts on page 17), Associate Justice Joyce L. Kennard writes the majority decision is "conspicuouslylacking in compassion," as the court rules an employee may be fired for "marijuana use, even when it occurs during off-duty hours, does not affect the employee's job performance, does not impair the employer's legitimate business interests, and provides the only effective relief for the employee's chronic pain and muscle spasms. I disagree."
Kennard writes that the majority opinion "disrespects the will of California's voters who, when they enacted the Compassionate Use Act, surely never intended that persons who availed themselves of its provisions would thereby disqualify themselves from employment."