Pharmacology. BAL (British anti-lewisite; dimercaprol; 2,3-dimercaptopropanol) is a dithiol chelating agent that is used in the treatment of poisoning by the heavy metals arsenic, mercury, lead, and gold. Because the vicinal thiol groups are unstable in aqueous solution, the drug is supplied as a 10% solution (100 mg/mL) in peanut oil that also contains 20% (200 mg/mL) benzyl benzoate. It is administered by deep IM injection. Most of the drug is absorbed within 1 hour and undergoes widespread distribution to most tissues. BAL, or its in vivo biotransformation product(s), is believed to form complexes with selected toxic metals, thereby minimizing the reaction of the metals with endogenous ligands and increasing their excretion in urine. In a study of humans treated with BAL after exposure to arsenicals, peak urinary arsenic excretion occurred in 2–4 hours and then declined rapidly.