The optimal dosage levels of CBD are uncertain due to a lack of human studies. Chronic high doses of up to 1500 mg per day are well tolerated and produce no noticeable physiological effects. However, there is evidence to suggest that the medical benefits of CBD disappear when dosages become excessive. For inhaled medical use, most users prefer varieties with THC:CBD ratios between 2:1 and 1:2. Extremely low-THC varieties are useful for making CBD extracts and tinctures. Unlike THC, CBD does not show up positive on standard drug tests for marijuana.