Which system was used for weighing drugs and solutions during the colonial period in the United States?

The apothecary system was the primary system used for weighing drugs and solutions during the colonial period in the United States. This historical system was specifically designed for use in pharmacies and medicine and it employs unique units of measure, such as grains, scruples, and drams, which were commonly used to denote quantities of medications and substances.

The apothecary system is notable for its focus on precision that is particularly critical in medicinal contexts, where accurate dosing can significantly impact patient health outcomes. This system not only included measures for weight but also volume, which were essential to the preparation and dispensing of medications in colonial apothecary practice.

While the metric and imperial systems have their own uses in modern settings, and the customary system refers to general measurement standards used in daily life, they were not the primary conventions employed by pharmacists or apothecaries in the colonial era for the measurement of drugs and solutions. Thus, the apothecary system stands out as the correct answer in this context.