Viagra is the brand name of a medicine men take for erection problems — when the penis will not become or stay hard enough for sex. It is a small blue tablet, and a doctor's prescription is needed for it. Because it was the first drug of its kind and was talked about everywhere in the news in the late 1990s, the brand name became the ordinary word: people say Viagra even when they mean any similar pill, in the same way people say 普拿疼 for any headache tablet. Two things about the word matter for writing. First, it is a name, so it takes a capital V. Second, it is used very often in jokes and comparisons, not only in medical talk — a newspaper might call something "Viagra for the economy", meaning something that suddenly gives strength or energy back to a thing that had gone weak. That figurative use is common and usually a bit playful or rude. In serious medical writing the drug's real chemical name is sildenafil, and doctors will use that. Socially the word is still slightly embarrassing to say out loud in Taiwan, so people often lower their voice or use the Chinese slang name.