A variety of drugs may occasionally cause transient crystalluria, in isolation or in conjunction with other urinary abnormalities. Overdose, dehydration, or hypoalbuminaemia, which increases the unbound drug which is ultrafiltrated by the glomerulus, are the factors usally favoring the precipitation of crystals within the tubular lumina. In some cases, medication-induced crystalluria has a distinct phenotypic appearance. For instance, Sulphadiazine crystals appear as strongly birefringent 'shocks of wheat' or 'shells' with an amber color. Acyclovir crystals are birefringent and needle-shaped, and when present in abundance give to urine a silky and opalescent macroscopic appearance. [from
HPO]