Background　Opioids are commonly used for relief of pain symptoms and for perioperative analgesic and sedation in tumor patients. However, the effects of opioids on tumor growth have been controversial.　Objective　To summarize recent literature about the effects of opioid reagents on tumors and the potential underlying mechanisms.　Content　Opioid drugs regulate the cell growth, metastasis, and tissue invasion of lung cancer, breast cancer, and other types of tumors. But whether the regulation is facilitation or inhibition depends on various factors, including subtypes and levels of opioid receptors, dosage of opioid drugs, the division stages of tumor cells, in in vivo or in vitro situations, and polymorphism of opioid receptor genes in patients, etc. Evidence shows that following activation of opioid receptors, diverse signal transduction pathways are activated, and may enhance or attenuate tumor growth.　Trend　Further investigations are required to clarify the effects of doses of opioid drugs on various tumor preparations and to elucidate their underlying molecular mechanisms. These studies will benefit the treatment of tumor with opioid drugs, not only for relieving pain symptoms, but also for reversing or controlling their potential facilitation of tumor growth.