THC and its major (inactive) metabolite, THC-COOH, can be measured in blood, urine, hair, oral fluid or sweat using chromatographic techniques as part of a drug use testing program or a forensic investigation of a traffic or other criminal offense.[52] The concentrations obtained from such analyses can often be helpful in distinguishing active use from passive exposure, elapsed time since use, and extent or duration of use. These tests cannot, however, distinguish authorized cannabis smoking for medical purposes from unauthorized recreational smoking.[149] Commercial cannabinoid immunoassays, often employed as the initial screening method when testing physiological specimens for marijuana presence, have different degrees of cross-reactivity with THC and its metabolites.[150] Urine contains predominantly THC-COOH, while hair, oral fluid and sweat contain primarily THC.[52] Blood may contain both substances, with the relative amounts dependent on the recency and extent of usage.[52]
CBD was first discovered in 1940 by Roger Adams, a prominent organic chemist at the University of Illinois. Shortly thereafter, other scientists began testing isolated cannabinoids on lab animals; notably, Walter S. Loewe ran trials on mice and rabbits with the cannabinoids THC, CBD and CBN. He found that CBD produced no observable effects in the animals’ behavior while THC caused, what he called, a “central excitant action” in rabbits. Despite science’s movement forward, scientists were completely unaware of the cannabinoids’ chemical structure, so no one could tell which specific compound resulted in which effect.
The author of a Harvard-led systematic review of 28 studies examining the efficacy of exo-cannabinoids (cannabinoids formed outside of the body, i.e. from the plant or synthetically made) to treat pain and other medical issues concluded, “the use of marijuana for chronic pain, neuropathic pain, and spasticity due to multiple sclerosis is supported by high-quality evidence.”
In this report, researchers reviewed 16 previously published studies testing the use of various cannabis-based medicines in the treatment of chronic neuropathic pain and found some evidence that cannabis-based medicines may help with pain relief and reduce pain intensity, sleep difficulties, and psychological distress. Side effects included sleepiness, dizziness, mental confusion. The authors concluded that the potential harm of such medicines may outweigh their possible benefit, however, it should be noted that the studies used a variety of cannabis-based medicines (e.g. inhaled cannabis and sprays and oral tablets containing THC and/or CBD from plant sources or made synthetically), some of which are more likely to result in these side effects than products without THC. 

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