This is required by the Drug Supply Chain Security Act (DSCSA) which applies to the USA. Pharmaceutical companies producing for export and pharmaceutical contract manufacturers must comply with DSCSA regulations in order to continue selling their products in the world's largest market for pharmaceutical products.
Since 30 June 2017 an important change has applied: In a draft directive issued by the FDA to manufacturers and other stakeholders in the supply chain for prescription-only drugs, it is explained that the prosecution and sanctioning of non-compliance with the statutory requirements, which was due to take effect from the changeover date in November 2017, will be suspended by one year. The changeover date originally scheduled will remain but the FDA will grant a transitional period until 11/2018 during which no sanctions will be imposed. The reason stated for the change is that manufacturers are to be given more time to implement the serialization requirement technically. Postponing the date is also intended to prevent disruptions to the supply chain.