Nystatin is a polyene antifungal antibiotic, commonly prescribed to treat yeast infections caused by Candida species, such as oral thrush or skin candidiasis. This drug works by binding to sterols in the fungal cell membrane, which disrupts the membrane’s structure and allows the cell’s contents to leak out, effectively killing the fungus. Because nystatin is sensitive to environmental factors and is used in different forms, understanding how long it remains effective and safe after opening is important for successful treatment. The usable life of nystatin once its seal is broken depends heavily on the specific formulation and how it is stored.
Shelf Life Varies by Formulation
The post-opening lifespan of nystatin differs significantly based on whether the product is a liquid, cream, or powder. Nystatin oral suspension, which is a liquid used to treat infections in the mouth, is the most fragile formulation and has the shortest usable life. The liquid is an aqueous suspension, meaning it is highly susceptible to chemical degradation and potential microbial growth once the bottle is opened.
Manufacturers’ instructions often specify that the oral suspension should not be used beyond a certain period, which can range from 7 days to as long as 30 days after opening, depending on the specific product and its preservatives. The presence of water in the suspension accelerates the decomposition of nystatin, and heat, light, and oxygen all contribute to a loss of activity over time.
Topical preparations, such as creams, ointments, and powders, generally maintain their integrity for a much longer duration after the initial use. These formulations are more stable because the active ingredient is protected within a semi-solid or dry base, which limits the exposure to moisture and oxygen. A nystatin cream or ointment often remains effective until the original printed expiration date, provided the product is stored correctly and kept free from contamination.
The key risk for topical nystatin is not primarily chemical breakdown but rather contamination from repeated use. If the product is in a tube, it is typically safe for three months once opened, while products in tubs may have a shorter advisory expiry date of one month due to the higher risk of being contaminated by fingers. Despite general guidelines, the precise discard date is always specified on the packaging insert or provided by the dispensing pharmacist, and this instruction should always be followed.
Storage Factors That Impact Stability
The chemical structure of nystatin is inherently sensitive, making its stability easily compromised by environmental exposure. Heat is a major factor that accelerates the decomposition of the drug’s active ingredients. Nystatin oral suspension is typically recommended to be stored at room temperature, generally between \(15^{\circ}\text{C}\) and \(30^{\circ}\text{C}\). Storing medication in areas that experience large temperature fluctuations, such as a bathroom medicine cabinet, can significantly shorten its effective lifespan.
Exposure to light, particularly direct sunlight, also causes the nystatin molecule to photodegrade, reducing its potency. Most nystatin products are therefore packaged in opaque or amber-colored containers to protect the contents from light. Keep the medication in its original container and away from windows or bright artificial light sources to maintain its strength.
Moisture and humidity pose a threat to all forms of nystatin, especially powders and the aqueous suspension. Excessive humidity can cause dry powders to clump or break down, while moisture can further destabilize the liquid form. Keeping the container tightly closed after each use minimizes exposure to both air and environmental moisture. Storing the medication in a cool, dry place, away from steam-filled environments like kitchens or bathrooms, helps preserve its therapeutic efficacy.
Recognizing When Nystatin is Unsafe to Use
Chemical and microbial changes in nystatin products often manifest as observable signs of degradation. For the oral suspension, a change in color, increased cloudiness, or the presence of large clumps that do not easily mix back into the liquid after shaking indicate that the medication is no longer safe or potent. A foul or unusual odor is another sign of potential spoilage or contamination.
Topical nystatin products also show physical changes when they have degraded. A cream or ointment may show a separation of its components, such as the oily and watery parts splitting apart, or the texture may become gritty or hardened. Using a product with these characteristics should be avoided, as the active drug may no longer be evenly distributed or present in a high enough concentration.
The primary risk of using compromised nystatin is treatment failure because the drug’s potency has been reduced. A secondary, and more serious, risk is the use of a contaminated product, particularly liquid suspensions, which can support the growth of other microbes that could cause a new infection or an adverse reaction. If any doubt exists about the integrity, appearance, or smell of the medication, it should be safely discarded, and a pharmacist should be consulted for a replacement.