Final answer:
The best action for the pharmacist is to contact the physician to verify the prescription for promethazine with codeine that was signed only by a PA, ensuring adherence to regulations.
Step-by-step explanation:
The pharmacist's best course of action when receiving a security prescription for promethazine with codeine signed only by a physician's assistant (PA) is to contact the physician to verify the prescription. Given that promethazine with codeine is a controlled substance, ensuring that the prescription is valid, including the proper authorizing signature, is critical for compliance with regulations and patient safety. Pharmacists, equipped with expertise in pharmacy and state licensure, are responsible for upholding the highest standards of drug dispensing and management.