Drug Toxicity Ratings Medications are classified according to how much risk they pose to a developing baby. (The higher the letter, the greater the risk.) Toxicity Rating Risk A Drugs, which are not toxic to humans. B Drugs, which does not show any significant risk or adverse effects in animal or human studies. C Drugs in which no adequate animal or human studies are carried out, or adverse effects are shown in animal studies and no human study data available. D Drugs with evidence of risk to the fetus but benefits outweigh the risk of the drug. X Drugs with proven risk to the fetus. Published on Apr 22, 2016Last Updated on Apr 22, 2016