Stifel analyst Jonathan Block downgraded Zoetis to Hold from Buy with an unchanged price target of $95. The animal health company closed yesterday down 8c to $92.09. While management execution has been "near flawless," a greater percentage of Zoetis' revenue growth now comes from a smaller number of therapeutics, Block tells investors in a research note. These therapeutics may not be approaching peak sales, but their contribution to growth likely diminishes going forward, the analyst adds. Further, he believes a "material product cycle" is likely 18 months away and that shares of Zoetis are trading at "peak relative valuation." Relative to the S&P 500, the stock is trading at a 57% premium versus its three year average of 37%, Block contends.