medicine, drug, dose
劑 centers on a measured medicinal preparation: a drug, medicine, or dose. The idea is a substance prepared in a specific amount for healing or treatment.
劑 combines 齊 (even, uniform) with 刂 (knife), suggesting cutting or preparing something evenly, which fits the idea of a measured medicinal dose. The modern simplified form is 剤.
The left side 齊 suggests uniformity and evenness, while the right side 刂 is a knife. Imagine a pharmacist using a knife to carefully cut and measure uniform doses of medicine.
For ザイ, picture a pharmacist preparing a dose of medicine while humming a tune: 'Zai zai zai…' The sound 'zai' cues ザイ as the medicine is being measured.