Certified Medical Assistant (CMA AAMA)PharmacologyMedium
A medical assistant is preparing to administer a viscous intramuscular penicillin injection to an adult patient. Which needle gauge is most appropriate?
- A18 gauge
- B27 gauge
- C21 gauge
- D25 gauge
Show answer & explanationAnswer & explanation
Correct answer: C. 21 gauge
Viscous medications require a larger-diameter (lower gauge number) needle to allow the thick solution to pass through easily; 21-23 gauge is standard for IM injections of viscous drugs like penicillin. Gauges 25-27 are too fine and may clog or bend, while 18 gauge is typically reserved for drawing up medication or IV access, not IM injection.
Why the other options are wrong
- A. 18 gauge is too large for routine IM injection and is more commonly used for withdrawing medication from vials.
- B. 27 gauge is used for very small volumes such as intradermal injections, not viscous IM injections.
- D. 25 gauge is too narrow and may not allow a thick solution to pass smoothly.
Needle Gauge Selection
Gauge number is inversely related to needle diameter; lower numbers indicate larger bore needles suited for viscous or larger-volume medications.
- 18-20 gauge: drawing up medication or thick solutions
- 21-23 gauge: standard IM injections
- 25-27 gauge: subcutaneous or intradermal injections
Memory trick: 'Lower gauge, bigger hole, thicker flow'