I was denied coverage for my substance use treatment with my insurer saying it is not medically necessary. When I asked for the criteria to determine whether a treatment is medically necessary, I was told this information is proprietary. Is this allowed?

No, this is not allowed. Your insurer must provide criteria for making medical necessity determinations, as well as any processes, strategies, evidentiary standards, or other factors used in developing any underlying non-quantitative treatment limitation (NQTL) such as medical management or prior authorization. If your insurer is not providing this information upon request, contact your local office of the Employee Benefits Security Administration, U.S. Department of Labor via the information here. (29 C.F.R. § 2590.712(d)(1); 45 C.F.R. § 146.136(d)(1); CMS, ACA Implementation FAQs – Set 29, Oct. 23, 2015.)