dose, medicine, drug
剤 centers on a measured amount of a medicinal substance: a dose of medicine or a drug. The idea of a prepared, cut portion connects the meanings.
剤 is the modern simplified form of 劑. The older form combines 刀 (knife) with 齊 (even, uniform), suggesting cutting and preparing a uniform mixture, which fits the idea of a measured medicinal dose.
The left side 斉 suggests something even or uniform, and the right side 刂 is a knife. Imagine a pharmacist using a knife to cut and prepare an even dose of medicine.
For ザイ, picture a pharmacist measuring out a dose of medicine and saying 'This is the exact size (サイズ) you need.' The 'size' sound cues ザイ.
medicine; agent; (chemical) substance; drug; dose
pharmacist; chemist
detergent; cleanser; cleaning agent; washing material
medicine; drug
pill; lozenge; tablet
formulation (of pharmaceuticals); preparation; pharmaceutical production
stimulant (e.g. psychoactive drugs like methamphetamine, ritalin, etc.)
making up a prescription; dispensing a prescription; preparing a medicine
solvent; solution; flux
laxative; purgative; evacuant
emulsifier
nutrient; nutritional supplement; nutritional tonic
fever medicine; antipyretic; antifebrile
anticancer drug; anticancer agent
medicine for treating digestive disorders (e.g. antidiarrheals, probiotics); gastrointestinal drug
rat poison; rodenticide
liquid medicine
(medicinal) iron preparations
antihistamine
pH adjuster