A competitive immunoassay method of detecting or determining phenethylamines of i) the 2C sub-family or ii) the DO sub-family, where a detecting agent competes with phenethylamines for binding with an antibody specific for phenethylamines of the 2C or DO sub-family, the method comprising: a) contacting a solution, or an in vitro sample taken from an individual suspected of containing phenethylamines, with one or more detecting agents comprising a phenethylamine substructure conjugated to an enzyme or fluorescent molecule and, for i) an antibody specific to phenethylamines of the 2C sub-family that does not cross-react with phenethylamines of the DO sub-family, or for ii) an antibody specific to phenethylamines of the DO sub-family that does not cross-react with phenethylamines of the 2C sub-family; b) measuring the signal or signals produced by the one or more detecting agents; and c) deducing from a calibration curve the presence of, or amount of, phenethylamines of the sub-family 2C, the sub-family DO or both the 2C and DO sub-families, wherein the antibody specific to phenethylamines of the 2C sub-family is raised against an immunogen derivatised through the 4-position of 2,5-dimethoxyphenethylamine and the antibody specific to phenethylamines of the DO sub-family is raised against an immunogen derivatised through the 4-position of 2,5-dimethoxy-N-methylphenethylamine; wherein the antibody specific to phenethylamines of the 2C sub-family is raised against an immunogen of structure I in which Q is H and the antibody specific to phenethylamines of the DO sub-family is derived from an immunogen of Structure I in which Q is CH 3 wherein, R 1 is H and and R 2 is where X is -(alk) n1 - n2 - in which A is O, S, or -N -, alk is C 1 - 6, substituted or unsubstituted straight chain alkylene or arylene moiety and n1 and n2 are independently 0 or 1; Y is a crosslinking group and n=0 or 1; Z is an antigenicity conferring carrier material; in which optionally or alternatively, n1 and n2=1, A is-N -, alk is a terminally substituted alkylene, optionally -S- CH 2 -CH 2 -C -; Y is 4-(succinimido-N-methyl)cyclohexylcarbonyl; and Z is bovine serum albumin; wherein the antibody specific to phenethylamines of the DO sub-family does not cross-react with phenethylamines of the 2C sub-family, wherein the antibody shows a cross-reactivity of less than 5%, optionally, less than 1% for 1-(4-bromo-2,5-dimethoxyphenyl)ethan-2-amine (2C-B), when compared to 100% for 1-(4-bromo-2,5-dimethoxyphenyl)propan-2-amine; and wherein the antibody specific to phenethylamines of the 2C sub-family does not cross-react with phenethylamines of the DO sub-family, wherein the antibody shows a cross-reactivity of less than 5%, optionally, less than 1% for 1-(4-bromo-2,5-dimethoxyphenyl)propan-2-amine, when compared to 100% for 1-(4-bromo-2,5-dimethoxyphenyl)ethan-2-amine (2C-B). a) contacting a solution, or an in vitro sample taken from an individual suspected of containing phenethylamines, with one or more detecting agents comprising a phenethylamine substructure conjugated to an enzyme or fluorescent molecule and, for i) an antibody specific to phenethylamines of the 2C sub-family that does not cross-react with phenethylamines of the DO sub-family, or for ii) an antibody specific to phenethylamines of the DO sub-family that does not cross-react with phenethylamines of the 2C sub-family; for i) an antibody specific to phenethylamines of the 2C sub-family that does not cross-react with phenethylamines of the DO sub-family, or for ii) an antibody specific to phenethylamines of the DO sub-family that does not cross-react with phenethylamines of the 2C sub-family; b) measuring the signal or signals produced by the one or more detecting agents; and c) deducing from a calibration curve the presence of, or amount of, phenethylamines of the sub-family 2C, the sub-family DO or both the 2C and DO sub-families, where X is -(alk) n1 - n2 - in which A is O, S, or -N -, alk is C 1 - 6, substituted or unsubstituted straight chain alkylene or arylene moiety and n1 and n2 are independently 0 or 1; Y is a crosslinking group and n=0 or 1; Z is an antigenicity conferring carrier material; in which optionally or alternatively, n1 and n2=1, A is-N -, alk is a terminally substituted alkylene, optionally -S- CH 2 -CH 2 -C -; Y is 4-(succinimido-N-methyl)cyclohexylcarbonyl; and Z is bovine serum albumin; wherein the antibody specific to phenethylamines of the DO sub-family does not cross-react with phenethylamines of the 2C sub-family, wherein the antibody shows a cross-reactivity of less than 5%, optionally, less than 1% for 1-(4-bromo-2,5-dimethoxyphenyl)ethan-2-amine (2C-B), when compared to 100% for 1-(4-bromo-2,5-dimethoxyphenyl)propan-2-amine; and wherein the antibody specific to phenethylamines of the 2C sub-family does not cross-react with phenethylamines of the DO sub-family, wherein the antibody shows a cross-reactivity of less than 5%, optionally, less than 1% for 1-(4-bromo-2,5-dimethoxyphenyl)propan-2-amine, when compared to 100% for 1-(4-bromo-2,5-dimethoxyphenyl)ethan-2-amine (2C-B).