Since hyaluronic acid is found within the body and is naturally produced in the cell walls, it is biocompatible and rarely causes any adverse effects. In products made from animal sources, the potential for an allergic reaction exists, but is unlikely.[1][2] One potential drawback of endogenous HA is that cancer cells may also produce high-molecular-weight HA. In these cells, the regenerative properties of HA support tumor growth by improving blood supply and can even protect the tumor from anticancer drugs. Some cancer treatments therefore include hyaluronidase to break down HA so that the medication can reach the tumor.[2] At very low molecular weights, measured as 40 KDa and below, HA can be a sign of stress and may be associated with inflammation.[1]