Marijuana is classified by the Drug Enforcement Agency (DEA) as Schedule I, drugs having no accepted medical value. Twenty-three states and the District of Columbia have legalized medical marijuana. This conflict inhibits physicians from prescribing marijuana and the systematic study of marijuana in medical care. This study concerns the use of the clinical laboratory as a resource for physicians recommending cannabidiol (CBD) to patients, or for patients using medical marijuana. Marijuana containing delta-9-tetrahydrocannabinol (THC) is psychoactive. CBD is not psychoactive. CBD is reported to have medical benefit for seizure control, neurologic disorders including multiple sclerosis, neuropathic pain and pain associated with cancer. Use of opiates leads to increasing dosage over time that may cause respiratory depression. The Medical Board of California has termed this a serious public health crisis of addiction, overdose, and death. Is it feasible that CBD might alleviate persistent, severe pain and therefore diminished opiate use? Further study is needed to determine medical effectiveness of CBD including the effect on concurrent opiate therapy due to competition for receptor sites. This study is the application of a gas chromatography mass spectrometry procedure adapted for use in our laboratory, to detect CBD in urine. The intended use is as a tool for physicians to assess that marijuana being used by a patient is of a composition likely to be medically effective. A law ensuring physicians freedom from federal prosecution would provide confidence essential to formal study of medical uses of marijuana and treatment of clinical problems. Detection of CBD in a urine sample would be a convenient test for such confirmation.