This is one of Rudyard Kipling's most famous quotations, and it is from a speech he made to the Royal College of Surgeons in London in 1923. Here, he is using the metaphor of comparing words to drugs to describe the persuasive effect words can have on another person. As he says in the next sentence of the speech, "Not only do words infect, egotize, narcotize, and paralyze, but they enter into and colour the minutest cells of the brain. . . ." He is describing the ability of a person to use words to change the way another person thinks and feels, to influence that person to do or feel things that are not normal for that person — just as drugs would do.
Perhaps when Kipling said this, he was thinking of what was going on in Europe during the 1920s, which saw the rise of the Nazi party in Germany in 1920 to threaten the security of all of Europe. Adolf Hitler had used his ability to persuade people with his words to take control of the Nazi party in 1921, and Hitler's book Mein Kampf
would become one of the most influential documents in the 20th century, serving as the cornerstone of Nazism and German socialism.