Which of the following medications requires refrigeration after opening?

Amoxicillin suspension requires refrigeration after opening due to its specific formulation and stability requirements. Once opened, the suspension is susceptible to microbial growth and can degrade over time if not stored properly. Refrigeration helps to maintain the integrity of the medication for an extended period, ensuring its effectiveness and safety for the patient.

In contrast, the other medications listed do not require refrigeration. Omeprazole tablets, simvastatin tablets, and calcium carbonate chewables are stable at room temperature and do not have the same risks associated with microbial contamination or degradation that necessitate cold storage. This understanding is crucial for proper medication storage and patient safety.