Although factors such as idiosyncratic reactions or allergic reactions can contribute to the overall safety profile of a drug, the therapeutic index gives a reliable indication of the drugs safety margin if other considerations are assumed to be equal. The therapeutic index represents the ratio between the median lethal dose (LD50) and the median effective dose ( ED50) of a drug. This calculation of LD50/ DD50 generates an index that is the therapeutic window between the dose that produces the desired effect and the dose that results in toxic effects. Like all indices, the T.I. is dimensionless since the dose units, usually express in "mg-kg ", cancel each other. The therapeutic index conveniently represents the safety merger of a drug. The greater the therapeutic ratio, or spread, between the ED50 and LD50, the greater the drug safety. Therefore a T.I. of one thousand represents a drug with a far greater safety profile than a drug with a T.I. of two. Ref: Von Zastrow M. (2012). Chapter 2. Drug Receptors & Pharmacodynamics. In B.G. Katzung, S.B. Masters, A.J. Trevor (Eds), Basic & Clinical Pharmacology, 12e.