Opiates such as morphine and codeine are substances often misused to improve the performance of racing horses during competitions and are therefore on the International Federation for Equestrian Sports' list of banned supstances. However, a positive antidoping test may be due to the consumption of feed (mainly alfalfa or oats) contaminated by poppy seeds containing the alkaloids morphine and codeine. In order to determine whether a positive antidoping test is the result of an intentional abuse of the opiates or consumation of feed contaminated by poppy seeds, it is necessary to develop a sensitive and specific analytical method for determining the mass fraction of morphine and codeine in horse feed. The aim of this study was to optimise conditions for the extraction of morphine and codeine from dehydrated alfalfa and develop and validate a gas chromatograhic-mass spectrometric method for the simultaneous determination of both analytes. The most efficient extraction of morphine and codeine from dehydrated alfalfa was achieved using a citrate buffer pH4 followed by solid phase extraction. The method showed good linearity (R2>0.9980) in the tested concentration range (50-1600 ng/g), as well as good precision (RSD<4 %), accuracy (95.5-97.7 %), and sensitivity (limit of detection=22 and 25 ng/g for morphine and codeine, respectively). The proposed method was used for analysing a sample of dehydrated alfalfa containing 1510 ng/g morphine and 327 ng/g codeine. Using this analytical method will enable the exclusion of horse food as a cause of positive antidoping tests.