Opoids and especially fentanyl have no direct effect on immune cells but on opoid receptor carrying stem cells and tumor cells which could affect various aspects such as angiogenesis and immune regulation by activating signal pathways of PI3K (phosphatidyl-inositol-3-kinase), Akt (proteinkinase B), and mTOR (mammalian target of rapamicin), and thereby promote tumor recurrence and metastasis [72]. Here, MTOR is linked to neoplasm.