Particulate matter, in both parenteral and ophthalmic drug products, consists of extraneous mobile, undissolved particles, other than gas bubbles, unintentionally present in the solutions. These particles can originate from various sources including, but not limited to: the environment, the manufacturing process, packaging materials and undissolved formulation ingredients. This particulate matter can be harmful when introduced to the body. Therefore, the US Pharmacopeia places limits on the amount of sub-visible particles that are allowed in drug products based on these risks.

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