Hyaluronic acid (HA) can also be injected into knee joints, but HA is a bit more contentious. HA is a very long-chain molecule, which makes it very oily and viscous. When it first became available, there were two main companies selling it; one sold it as a ‘medical product’, saying that it acted as a mechanical lubricant, effectively ‘oiling’ the knee joint. The other company registered it as a drug, saying that it had a pharmaceutical effect in the knee, like a painkiller. Either one of these companies was right and the other wrong, or maybe both were a bit right, or maybe they are both wrong?!