The majority of the association studies analyzing OPRM1 and drug addiction have focused on common variants. Common variants have an allele frequency greater than 5% in the general population and when associated with disease typically confer a small to moderate amount of risk. Most genetic variants associated with drug addiction-related phenotypes have odds ratios (OR) in the range of 1.1–1.3. Conversely, the rare variant hypothesis states that a significant proportion of disease risk arises from low frequency variants (<1%) that confer a much greater risk for disease. The mean OR for rare variants across a range of common diseases is estimated to be 3.74 [8].
