Lipinski's rule of five also known as the Pfizer's rule of five or simple rule of five (RO5) is rule of thumb to assay drug-likeness or regulate if a chemical compound with a positive pharmacological or biological activity has chemical properties and physical properties and would make it a likely orally active drug in humans. The rule was formulated by Christopher A. Lipinski in 1997, based on the observation that most orally administration drugs are relatively small and slightly lipophilic molecule. The rule states that poor absorption or infiltration of drug more reasonable when the chemical structure fulfils two or more of the following criteria: