膏 usually appears in compounds: 牙膏 (toothpaste), 药膏 (ointment), 膏药 (medicated plaster), 润肤膏 (skin cream). Standalone 膏 is literary.
history
In classical Chinese, 膏 referred to animal fat, and by extension to anything rich or fertile: 民脂民膏 (mínzhī mín gāo) means 'the people's blood and sweat' (fat and grease), referring to wealth extracted from common people.
