Morphine's actions outside the brain.

Morphine is well recognized as an addictive narcotic whose effects on the brain lend it remarkable pain-killing properties. Less appreciated is the drug's ability to weaken the body's defenses. "It clearly appears that, by a number of parameters, the immune system is depressed," says Toby K. Eisenstein of Temple University School of Medicine in Philadelphia.

Various immune cells bear opioid receptors, the cell surface proteins that morphine uses to convey signals into cells, notes Eisenstein. Moreover, spleen cells taken from mice receiving morphine mount a weaker-than-normal immune response in test-tube studies. Eisenstein says her work suggests that the drug impairs the ability of immune cells called macrophages to communicate with T cells, another arm of the immune system.

Eisenstein has also shown that mice receiving morphine regularly die from bacterial infections that would not be lethal to normal mice. She cautions that patients receiving morphine for postsurgical pain may face increased risk of sepsis, a massive, often fatal bacterial infection.