Final answer:
The nurse should stop the penicillin G injection and inform the healthcare provider of the patient's reported allergy to amoxicillin. The patient's allergic reaction indication requires immediate attention, careful documentation, and consideration of alternative antibiotic treatments.
Step-by-step explanation:
If an 18-year-old patient expresses a concern about a potential allergic reaction to penicillin based on a previous experience with amoxicillin, the nurse's immediate next course of action should be to halt the administration of penicillin G. Because amoxicillin is a derivative of penicillin, patients who have had allergic reactions to amoxicillin could potentially be allergic to other penicillin derivatives as well. The nurse should notify the prescribing physician or healthcare provider about the patient's self-reported allergic reaction to amoxicillin immediately. In the meantime, close monitoring of the patient for signs of an allergic reaction is crucial. Appropriate documentation of the reported allergy should be made in the patient's medical chart for future reference.
Subsequently, the healthcare provider may assess the situation, potentially order allergy testing, and choose an alternative antibiotic medication if necessary, such as a non-penicillin antibiotic, to treat the streptococcal pharyngitis and tonsillitis.