Free radicals are very destructive to the body, and eating free radicals in food could cause lipid peroxidation and other damage in important organs. Free radicals could react with and use up vitamin antioxidants in food, altering nutritional content. Free radicals could cause mutations in intestinal cells, leading to cancer. Free radicals could react with food components to create unusual, possible toxic biochemical products. We don't know which animal will respond most like a human to a given drug, so we need to try a variety.
Drug metabolism tends to be faster in smaller animals.
Animals are 10 times less susceptible to toxicity than humans. One popular folk remedy claims that drinking apple cider vinegar when eating suspicious-looking food protects against food poisoning. Perhaps this is an exaggeration, but consider whether there is any basis to this statement. Stimulation of estrogen receptors in post-menopausal women and other patients suffering from estrogen deficit. Blockage of estrogen receptors in obese patients and others whose whose endogenous levels of estrogen are exceptionally high.