Figure 3.

Hormetic effects of low-level light therapy (LLLT). LLLT does not induce classical linear dose-response pharmacological effects. LLLT effects are characterized by inverted U-shaped dose-response curves, in which linear responses may be seen only at very low doses. Whereas linear effects may be negligible, maximal stimulatory effects are typically observed at intermediate doses. However, the linear relationship does not hold at high doses, since inhibitory effects are observed instead. In fact, the inhibitory effects of very high LLLT doses might be worse than control conditions (eg, tissue destruction). A key observation concerning the modulatory effects of light in tissues is that maximal responses at intermediate doses tend to represent less than twofold increases in biological variables relative to baseline conditions. Yet these effects have been shown to have major relevance, especially when energy metabolism is involved in nervous tissue. Thus, hormesis is an essential concept for the development of neurotherapeutic applications of LLLT.