Horowitz was treated with primitive tricyclic antidepressants. I was given a muscle relaxant that is closely related to those and it was the worst prescription drug experience of my life. How he even managed to perform while using those is beyond me.
I am wary of the claim that he was always crazy. My guess is that the strain of being gay, living away from his native land, being part of a second persecuted group (particularly in his native land), and having to be married to a woman he had to speak French with as they had no other common language — was a lot. Add to that his musical genius and the incredible demands his early performative skills placed on him as he aged. Add to that that he was clearly a highly-competitive perfectionist with an immense depth of musical understanding (made clear by the fact that even performances he isn't renowned for, such as his in-home recording of Scriabin's 3rd sonata have more interpretative detail than any other recorded performance). Being a perfectionist is on its own is a strain. His daughter also suffered from bipolar disease or something similar and committed suicide.
That he was always headstrong doesn't seem to be much in dispute. He became famous first for his refusal to abide by the tempo of the conductor in a performance of the Tchaikovsky first concerto. He sped away from the conductor and the orchestra, doing as he pleased. He may have also been impetuous even as a young man. I haven't researched that time of his life much at all. I do know that later in life one of the men he had sex with regularly said he was prone to bursts of rage and would routinely dump the guy's meal in his lap during dinner. How much of that is due to insanity, though? How much is due to the effects of the tricyclics, which have a very controversial track record (see the Elavil controversy, for instance)? How much is from having to live a lie despite supposedly being adored by nations of musically-inclined people? That's pretty corrosive on one's being, that kind of hypocritical prison. And, also... there is also an entitlement that goes with being a 'maestro' coupled with the frustration of becoming old. Without a male spouse more emphasis is on one's looks to attract romantic interest. The more one's looks decline, the more the frustration will grow.
Interestingly enough, he described his meeting with the composer Scriabin, saying he (Scriabin) was clearly mad. However, the shaking movements Horowitz claimed were evidence of madness were probably due to a different health problem, perhaps one linked to his early death. There is also a rumor that his son was murdered (which happened after his death). In the case of Van Gogh, it is now believed that syphilis played the largest role in his cognitive decline. I think the effects of arsenic in particular (from the green paint he used) is underestimated. His palette was a soup of toxic elements and urban places of that time in particular were often full of deadly toxins. Most of the 'artistic temperament' in terms of artists of his period is related to such poisoning. Cezanne's irritability and diabetes are both symptoms of chronic arsenic exposure. Paris green was his favorite paint pigment. Pianos were treated with dust that was a mixture of arsenic and lead, to prevent insects from feeding on the felt. I don't know if the felt was also processed with mercury.
