According to the USP, zinc oxide ointment should contain not less than and not more than zinc oxide.

• A. 18.5%, 21.5%
• B. 18%, 22%
• C. 20%, 25%
• D. 15%, 25%

Answer: (A)

The principle being tested is how USP sets a potency specification for a medicated ointment. For zinc oxide ointment, the active ingredient must be present in a narrow, defined range by weight to guarantee consistent therapeutic effect and product quality. The USP monograph specifies not less than 18.5% and not more than 21.5% zinc oxide. This keeps the product near the common labeling strength (around 20%) while allowing for analytical and manufacturing variation. If the content were below the lower limit, the ointment could be under-potent and less effective; if above the upper limit, it could alter physical properties like texture and stability and potentially affect safety. So the 18.5–21.5% range is the specification that aligns with the USP standard for zinc oxide ointment.