Express Scripts (ESRX) has announced it is not going to cover Teva's (TEVA) new migraine drug Ajovy, Wells Fargo analyst David Maris tells investors in a research note. The analyst views the decision as a "clear negative" for Teva saying its new migraine drug is its highest profile growth driver. Express Scripts' decision comes as the company is already facing a challenging generic drug market and a cost cutting program, Maris tells investors in a note titled "Continued Caution." He believes today's coverage decision also makes Teva's negotiations with other payors more likely to be challenging as it shows Novartis (NVS) and Lilly (LLY) are "playing and discounting to win." Maris points out that Express Scripts is estimated to cover approximately 100M lives in the U.S. The analyst sees potential downside to Teva's consensus estimates if Express Scripts' coverage decision on Ajovy is repeated with other major payors. He keeps a Market Perform rating on Teva. The stock in morning trading is down 6%, or $1.25, to $21.05.