Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:Moreover, variability in analgesic efficacy could be explained by inter-subject variations in plasma levels of parent drug and its active metabolite eventually affected also by co-medications [19], and therefore, dosage must be titrated for optimal effect and avoidance of toxicity after appropriate TDM.
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