According to the US Pharmacopeia, select the most common type/s of chemical reactions that cause most drugs to lose stability. Select all that apply

Drug stability refers to the capacity of a pharmaceutical formulation to remain within defined physical, chemical, and microbiological specifications. Chemical degradation typically involves the breaking or forming of covalent bonds, leading to a loss of potency or the formation of toxic byproducts. Environmental factors like temperature, moisture, and pH act as catalysts for these kinetic processes. Understanding these reactions is essential for determining expiration dates and storage requirements. Rationale: A. Photolysis is a degradation process triggered by the absorption of radiant energy or light. While significant for light-sensitive drugs like nitroprusside or nifedipine, it is not considered one of the two "most common" general pathways for the majority of pharmaceuticals. It requires specific chromophores to occur. Most drugs are primarily susceptible to moisture or oxygen before photo-degradation occurs. B. Oxidation-Reduction (redox) reactions involve the transfer of electrons and are a primary cause of drug instability. Many medications, particularly those with phenolic or ethereal functional groups, are highly sensitive to atmospheric oxygen. This process often leads to color changes and the production of free radicals. It is a dominant pathway for chemical degradation in many liquid and solid dosage forms. C. Hydrolysis is arguably the most common cause of drug instability, involving the cleavage of chemical bonds by water. Functional groups such as esters, amides, and lactams are particularly vulnerable to nucleophilic attack by water molecules. Since many drugs are stored in environments with humidity or formulated as aqueous solutions, hydrolysis is a constant threat to molecular integrity. It remains a critical concern in pharmacopoeial standards. D. Epimerization is a specific type of degradation where a molecule shifts the configuration of a single chiral center. While clinically relevant for drugs like tetracycline, where it leads to a loss of activity, it is a specialized reaction. It is not categorized by the USP as a universal mechanism of instability compared to hydrolysis. Its occurrence is limited to specific stereochemical structures. E. Isomerization involves the conversion of a drug into its optical or geometric isomer. Similar to epimerization, this results in a change in pharmacological activity but is not a ubiquitous degradation pathway. Most pharmaceutical instability issues across the broad spectrum of drug classes are rooted in redox or hydrolytic processes. Isomerization remains a secondary, less frequent concern in general stability testing.