A reaction where the total is more than the sum of the individual parts. For example, noise and certain drugs have a synergistic effect on hearing loss. Assume that a given amount of noise normally causes 1 unit of hearing loss. Also, assume that a given drug normally causes 2 units of hearing loss. Therefore, if you were exposed to both the noise and the drug, you would expect to have 3 units of hearing loss. However, in this hypothetical example, when tested, you find you have 7 units of hearing loss. The extra 4 units above what you would have expected by adding up the two figures, is caused by the synergistic action of noise and ototoxic drugs working together.