Insect can be used as alternative specimens for toxicological analysis when conventional post-mortem samples are not available as drugs that can be detected in severely decomposed tissues of a corpse may still be found in the insects that did feed on the corpse. Several studies illustrate the great potential importance of entomotoxicology for providing additional information on cause of death. This study was aimed to study the effect of intoxication with codeine phosphate in decomposed bodies on the development of carrion flies and on the evaluation of postmortem interval (PMI). The experiment was performed during summer season from last of June to the mid of July. Four rabbits, two control and two injected with lethal dose of codeine phosphate were used. Each one was placed in a cardboard box, floored with muddy soil and protected with a metal cage. Different developmental stages of Chrysomya albiceps were collected and studied for both biological and morphological changes using dissecting light microscopy. There were morphological changes in the form of disfiguring of segments, loss of colouration and abnormalities in the shape of both anterior and posterior spiracles in the larvae, while the adult flies show rudimentary wings, abnormal bands on the undersurface of the abdomen, fading of normal colour to complete loss of it. Also there were biological changes in the form of acceleration in the development during life cycle and incomplete emerge of some adult flies from their pupae in the injected group. These acceleration of development lead to bias in estimation of postmortem interval up to 24h when estimation based on larval development and 48h when estimation based on pupal development. This work should provide data for the use of morphological and biological changes that occur in Chysomya albiceps as indicator for death from lethal dose of codeine phosphate and to estimate the post mortem interval.