Synergistic effects

Joint effects of two or more agents, such as drugs that increase each other's effectiveness when taken together. A biologic response to multiple substances where one substance worsens the effect of another substance. The combined effect of the substances acting together is greater than the sum of the effects of the substances acting by themselves

http://rais.ornl.gov/home/glossary.html#L  
http://www.atsdr.cdc.gov/glossary.html#G-J -