In materials science, a disappearing polymorph is a form of a crystal structure that is suddenly unable to be produced, instead transforming into a different crystal structure with the same chemical composition (a polymorph) during nucleation.[2][3] Sometimes the resulting transformation is extremely hard or impractical to reverse, because the new polymorph may be more stable.[4] It is hypothesized that contact with a single microscopic seed crystal of the new polymorph can be enough to start a chain reaction causing the transformation of a much larger mass of material.[5] Widespread contamination with such microscopic seed crystals may lead to the impression that the original polymorph has "disappeared."

When the patent for paroxetine anhydrate (the "original" polymorph) ran out, other companies wanted to make generic antidepressants using the chemical. The only problem was that by the time other companies began manufacturing, Earth's atmosphere was already seeded with microscopic quantities of paroxetine hemihydrate from GSK's manufacturing plants, which meant that anyone trying to manufacture the original polymorph would find it transformed into the still-patented version, which GSK refused to give manufacturing rights for. As it is illegal to manufacture or sell anyone's patented product without their permission, GSK sued the Canadian generic pharmaceutical company Apotex (SmithKline Beecham Corp. v Apotex Corp) for patent infringement by producing quantities of the newer paroxetine polymorph in their generic pills, asking for their products to be blocked from entering the market.[10][7]