Anesthetics and opiate analgesics also cause NK cell suppression. Research in animals and in humans has shown that morphine and fentanyl cause suppression of NK cell activity, and enhance susceptibility to breast cancer metastases in rats (11,12). Lower doses of opiates in patients have been reported to decrease the immune suppression magnitude and duration (13). Therefore, peri-operative interventions which decrease the use of opiates (or their endogenous secretion) may be clinically important factors in cancer recurrence.