Xyzal is a common “over the counter”(OTC) drug (antihistamine) used for allergy relief. Some animal studies have suggested that antihistamines, especially H1 blockers, can affect the osteoblastic/osteoclastic activity of bone remodeling resulting in an inhibiting of the tooth movement process, the degree of which is undetermined.
With normal use as directed by the manufacturer and in the absence of an allergy to the drug itself, there does not appear to be notable health risks to taking the drug.
Orthodontic treatment for a patient taking antihistamines may take a little longer, but the normal use of the drug should not be a contraindication to the treatment. The wear schedule for a clear aligner patient may be lengthened a little, but this is not a requirement.