Final answer:
The volume limit for OTC Schedule III-V substances to purchasers 18 or older is 240 mL for non-opium products and 120 mL for opium products every 48 hours without a prescription.
Step-by-step explanation:
The volume limit for OTC products, Schedule III-V, to a purchaser who is 18 or older for non-opium containing products is 240 mL (8 oz.) of any controlled substance containing codeine per 48 hours without an individual prescription. For purchasing opium-containing products, the volume limit is 120 mL (4 oz.) per 48 hours without an individual prescription as these rules are established by the Controlled Substances Act and enforced by the Drug Enforcement Administration (DEA). These restrictions aim to prevent abuse and diversion of controlled substances while ensuring access for legitimate medical needs.