Chocolate contains substances known as methylxanthines (specifically caffeine and theobromine), which dogs are far more sensitive to than people. Different types of chocolate contain varying amounts of methylxanthines. In general, though, the darker and more bitter the chocolate the greater the danger. StartFragment [if IE]><link href="/themes/petmd_v3/css_v3/ie-bugfixes.css" rel="stylesheet" type="text/css"><![endif][if IE 7]><link href="/themes/petmd_v3/css_v3/ie7-bugfixes.