Searching for Synergism Among Combinations of Drugs of Abuse and the Use   of Isobolographic Analysis
  It is well known that individuals who abuse drugs usually use more than one substance. Toxic consequences of single and multiple drug use are well documented in the Treatment Episodes Data Set that lists combinations that result in hospital admissions. Using this list as a guide, we focused our attention on combinations that result in the most hospital admissions and searched the PubMed database to determine the number of publications dealing with these toxic combinations. Of special interest were those publications that looked for or used the term synergism in their titles or abstracts, a search that produced an extensive list of published articles. However, a further intersection of these with the term isobole revealed a surprisingly small number of literature reports. Because the method of isoboles is the most common quantitative method for distinguishing between drug synergism and simple additivity, the small number of investigations that actually employed this quantitation suggests that the term synergism is not properly documented in describing the toxicity among these abused substances. The possible reasons for this lack of quantitation may be related to a misunderstanding of the modeling equations. The theory and modeling are discussed here. 
Ronald J. Tallarida, Uros Midic, Neil S. Lamarre, and Zoran Obradovic
q-bio.QM