This species is known to produce the dermatotoxin lyngbyatoxin and to be the cause of "Swimmer's itch" when in direct contact with human skin.
Osborne, N. J., Webb, P. M. & Shaw, G. R. (2001). The toxins of <i>Lyngbya majuscula</i> and their human and ecological health effects. <em>Environment International.</em> 27(5): 381-392.