For decades, lithium carbonate has been a cornerstone treatment for bipolar disorder

and related conditions, yet its mechanism of action remains poorly understood.

Behavioural studies have suggested that the two stable isotopes, 6Li and 7Li, with

nuclear spins of 1 and 3/2, respectively, can produce distinct biological effects. Because

nuclear spin can subtly alter the course of specific chemical reactions, these observations

have motivated the exploration of whether lithium’s isotopes might act through a

spin-dependent mechanism. Here, we investigate the Radical Pair Mechanism in the

flavin–ascorbyl system. The ascorbyl radical, derived from vitamin C, is both abundant

in the brain and long-lived, making it a more credible partner for flavin. We propose

that lithium’s nuclear spin modulates triplet yields in flavin–ascorbyl radical pairs,

offering a quantum-based explanation for isotope-dependent effects. Whether these spin

interactions play a clinical role remains to be determined, but the possibility is exciting

and points toward a promising line of inquiry.