This section is from the book "Materia Medica And Therapeutics Inorganic Substances", by Charles D. F. Phillips. Also available from Amazon: Materia medica and therapeutics.
The therapeutical influence of phosphoric acid is mainly exerted on the nervous system, and in the treatment of nerve-debility acts much like iron in anaemia, as a chemical food supplying something actually deficient in nerve-nutrition. When mental effort has been protracted till a sense of weariness renders its continuance difficult, a dose of the acid, from its stimulant effect, relieves fatigue, and seems to invigorate the mental powers, and prepare the mind for renewed exertion.
Dr. J. Andrews, describing a case of impaired mental power from excessive brain-activity, observes, "The patient is languid, unable to do mental work with the usual facility, nervous, and at times fearful, timid, and agitated, the memory weakened, and permanent impairment threatened. Such cases have been termed ' cerebral paresis,' but for their recovery, relaxation from business, and phosphoric acid, with some suitable tonic, generally suffices."
Of more serious conditions, such as dementia following acute mania, Dr. Andrews remarks, "This is a period of nervous exhaustion, of reaction from the increased mental and physical activity which marked the previous state of the disease; tone and vigor must be supplied to the prostrated system, and for this phosphoric acid is of material service." It relieves peripheral congestions connected with impaired tone of vasomotor nerves, and in weakened relaxed conditions akin to impotence, and resulting from sexual excess, it has proved a special help.