The key is demonstrating that the product's commercial value derives from its isotopic enrichment rather than its base chemistry. A certificate of isotopic purity from the manufacturer, showing enrichment above natural abundance, supports classification here. If the compound is not isotopically enriched, it would normally fall under its base chemical heading in Chapter 28 or 29.

Stable isotope-labeled compounds used as active pharmaceutical ingredients or in clinical research may be subject to FDA review depending on their intended use. However, isotopes imported as research reagents or analytical standards generally enter as chemicals rather than drugs. End-use documentation clarifying research versus clinical application can help avoid delays at entry.