The biologically active agent in the cannabis plant tetra - hydro - cannibol is highly concentrated in the flowering tops . herbal cannabis below left is produced by drying the leaves for several days . dried cannabis can maintain its potency for several years 含丰富生物活性剂四氢大麻醇tetra - hydro - cannibol的大麻花冠,弄乾后便成为像大麻草般的绿色烟叶见左下图,大麻草的效力可维持数年之久。