This talks about Grant's addiction to cocaine as a way to deal with the pain of throat cancer to get his autobiography written. He, like many others of the time, was addicted through patented medicine of the time:
However at age 63, the two-term President wasn’t poking snot with a rolled up $50 bill. He ingested his blow via a “French wine tonic” called Vin Mariani. The coca leaf-spiked concoction fueled ol’ Lyss to finish what became an instant bestseller that was hailed by critics. Each day Grant went through multiple bottles, each containing six milligrams of cocaine per fluid ounce. He pretty much slept the rest of the time.
The New York Times reported multiple accounts of Grant’s doctor as his main supplier. On Easter Sunday 1885, the President’s throat “was gargled and dressed in cocaine.” On April 11, the Gray Lady noted the patient “has been ‘better’ at various times only in the sense that his decline has been stayed because of drugs.”
Given the amount that the man was ingesting, his physician likely wasn’t the only one providing the juice. Twain, whose publishing house was still in its first year, later admitted to aspirations of becoming a drug dealer. In his autobiography The Turning Point of My Life (1910), he shared ”a longing to ascend the Amazon. Also with a longing to open up a trade in coca with all the world.”