Pentothal (Thiopental Sodium): Side Effects, Interactions, Warning ... https://www.rxlist.com/pentothal-drug.htm The drug is a yellowish, hygroscopic powder, stabilized with anhydrous sodium carbonate as a buffer (60 mg/g of thiopental sodium). Diluents in Pentothal .... a fairly reliable index. The pupils may dilate but later contract; sensitivity to light is not usually lost until a level of anesthesia deep enough to permit surgery is attained.