Crystallization can occur for many reasons, depending on the matter. Influences like sunlight, temperature and stirring can cause crystallization of some materials. If you are using a professional form of medication, the crystallization should be abnormal and it'd be best to consult your pharmacist. The product leaflet might give you more information.
The crystallized product is obviously no longer in the state it was meant to be by the producer, so it is no longer safe to use.
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