In mammals, females occasionally nurse offspring that are not their own. The function of allosuckling for foster females remains puzzling given that lactation is energetically costly and augments the risk of pathogen transmission. Current hypotheses suggest that allosuckling is not adaptive resulting from misguided parental behaviour, females reciprocate by nursing each other's offspring, females nurse preferentially related offspring for inclusive fitness benefits, females involved in allosuckling events evacuate extra milk or improve their maternal skills. These hypotheses received only little empirical evidence, and a recent review suggested that they are not mutually exclusive and do not exclude alternatives. Based on the established fact that teat stimulation by neonates leads to an elevated production of prolactin, I develop the hypothesis called ‘neuroendocrine function of allosuckling’ (NFA). The hypothesis postulates that the nursing of alien offspring allows females to increase and/or maintain prolactin concentration in the case own offspring do not stimulate their teats enough. This neurohormone enhances immunocompetence, the immunological quality and the quantity of milk whilst reducing female's fertility. The NFA suggests that via teat stimulation by the genetic offspring, and if necessary by foster ones, females can optimally adjust prolactin concentration, as both hypo- and hyper-prolactinaemia entail physiological and immunological costs. The hypothesis does not only provide an explanation as to why females may nurse alien offspring but is also useful to understand the consequences of allosuckling. Indeed, even if mothers do not nurse alien offspring to control their prolactin production, allosuckling will alter the concentration of this crucial neurohormone.