Fortified antibiotic eye drops are the most well-known example of antimicrobial drugs compounded specifically to increase potency. Vancomycin and tobramycin are the two most frequently prepared, with pharmacies compounding them at concentrations several times higher than any commercially available product. This practice is standard in ophthalmology for treating serious corneal infections, but compounding to boost antimicrobial strength also extends to topical wound care formulations, otic preparations, and antifungal treatments.
Fortified Ophthalmic Antibiotics
When a bacterial infection threatens the cornea, commercially available antibiotic eye drops often don’t deliver a high enough drug concentration to the tissue. Pharmacies solve this by compounding “fortified” versions. Fortified vancomycin is typically prepared at 25 mg/mL, and fortified tobramycin at 14 mg/mL. By comparison, standard commercial tobramycin eye drops contain 3 mg/mL, making the fortified version nearly five times more concentrated.
This combination covers a broad spectrum: vancomycin targets gram-positive bacteria (like staph and strep species), while tobramycin handles gram-negative organisms. Together, they form the standard first-line treatment for bacterial keratitis, a potentially sight-threatening infection of the cornea. Because these fortified drops aren’t manufactured commercially, they must be compounded fresh by a pharmacy, usually at a hospital.
Storage matters with these preparations. Fortified vancomycin in particular loses potency at room temperature and should be kept refrigerated. Studies testing potency over 14 days found that vancomycin stored at cool temperatures maintained its strength, while samples kept at warmer temperatures degraded more quickly. Patients using these drops at home are typically instructed to store them in the refrigerator and discard them after a set period.
Why Compounding Increases Potency
Compounding to increase potency works in two main ways: raising the drug concentration beyond what commercial products offer, or using specialized vehicles and delivery systems that help the drug penetrate tissue more effectively. Both approaches aim to deliver a higher amount of active drug directly to the site of infection.
The vehicle, or base, that carries the drug plays a significant role. Emulsion-based systems, for instance, protect the drug from breaking down, improve its ability to dissolve in water, and increase how much of it cells actually absorb. Lipid-based and polymer-based carriers can target drug delivery more precisely, improving tissue and biofilm penetration while reducing side effects. One striking example: vancomycin attached to nanoparticle carriers showed 15- to over 100-fold stronger potency compared to conventional delivery, and this enhanced activity held even against vancomycin-resistant bacterial strains.
Topical Antifungal Compounding
Amphotericin B, a powerful antifungal, is another antimicrobial frequently compounded for topical use at concentrations not available in commercial products. Researchers have developed gel-based formulations using a hydrogel vehicle that can carry both water-soluble and oil-soluble substances. One well-studied preparation dissolves amphotericin B in a solvent, then incorporates it into a gel at a final concentration of 0.1%, creating a topical treatment for skin infections like cutaneous leishmaniasis that would otherwise require intravenous therapy.
The choice of gel base matters. A vehicle composed of fatty oil, a gelation-promoting polymer, and a non-ionic surfactant helps the drug penetrate the outer skin layers while remaining stable enough to maintain its potency over time. Without this kind of compounding, amphotericin B is poorly absorbed through the skin and would not reach therapeutic levels at the infection site.
Compounded Antimicrobials for Wound Care
Chronic wounds that resist healing often harbor biofilms, which are colonies of bacteria encased in a protective slime layer that standard antibiotics struggle to penetrate. Compounded topical antimicrobials address this by delivering higher local concentrations directly to the wound bed. Iodine-based formulations are commonly used in this setting. Cadexomer iodine products can destroy biofilms and the bacteria within them, partly by boosting the body’s own immune response at the wound surface, increasing the activity of infection-fighting white blood cells.
Silver-based compounds work through a different mechanism, destabilizing the protective outer layer of the biofilm itself. Composite wound dressings that release iodine in a sustained fashion over time can also be paired with hydrogel bases that may deliver oxygen to the wound, supporting the healing process alongside the antimicrobial effect.
Quality Concerns With Compounded Drugs
Compounding increases flexibility, but it also introduces variability. A study comparing compounded doxycycline tablets and chews to FDA-approved formulations found significant inconsistencies. The acceptable potency range for compounded products is 90% to 110% of the labeled amount. Some compounded tablets fell below this range at 89% on day one, while others exceeded it at 116%. Compounded chewable formulations performed worse: one product tested at just 81% potency on day one and dropped to 67% after 21 days of storage. Another started within range at 98% but fell to 69% over the same period.
FDA-approved formulations, by contrast, consistently fell within their acceptable range (90% to 120%) on both day one and day 21. This gap highlights a real trade-off. Compounding allows prescribers to create concentrations and formulations that don’t exist commercially, but the final product depends heavily on the compounding pharmacy’s quality control, ingredient sourcing, and storage practices.
How Compounding Is Regulated
In the United States, compounding pharmacies operate under two main regulatory frameworks. Section 503A of the Federal Food, Drug, and Cosmetic Act covers traditional pharmacies that compound drugs based on individual prescriptions. Section 503B covers outsourcing facilities that can compound larger batches without patient-specific prescriptions but must register with the FDA and follow stricter manufacturing standards, including current good manufacturing practices.
The FDA maintains a list of bulk drug substances that outsourcing facilities may use under section 503B. This list is updated periodically as the agency evaluates nominated substances on a case-by-case basis. Antimicrobials compounded under 503A (individual prescriptions) have more flexibility in what ingredients can be used, but they also face less regulatory oversight. For patients receiving compounded antimicrobials, the reputation and accreditation of the compounding pharmacy is a practical indicator of product quality.