Compounding (503A/503B)

Compounding is when a licensed pharmacy mixes or prepares a medication itself, rather than dispensing a mass-manufactured product straight from a drugmaker. It exists to fill real gaps: a patient who needs a different strength, a liquid instead of a pill, a formula without an allergen — or a drug that’s in shortage.

The two numbers you’ll see, 503A and 503B, refer to sections of federal law that split compounders into two lanes:

• 503A pharmacies compound for a specific patient with a prescription — one person, one preparation. This is the traditional corner-pharmacy sense of compounding.
• 503B “outsourcing facilities” compound in larger batches without a patient-specific prescription and can sell to clinics. In exchange, they follow stricter, more manufacturer-like FDA quality standards.

This distinction became a big deal with GLP-1 drugs. When semaglutide and tirzepatide were in official shortage, compounding pharmacies were allowed to make their own versions, which is why “compounded semaglutide” flooded the market at lower prices. When a shortage ends, that permission narrows sharply.

Here is a concrete example. A patient prescribed semaglutide may need a strength or ingredient combination the approved product does not provide, so a 503A pharmacy can prepare a patient-specific version when federal and state requirements are met. During a shortage, a 503B outsourcing facility may prepare batches of semaglutide or tirzepatide for clinics under the separate 503B rules. In both cases, the compounded preparation is distinct from the FDA-approved product: it has not gone through FDA approval as that finished drug, even though the pharmacy or outsourcing facility operates under legal compounding requirements.

The key thing to hold onto: compounded drugs are legal and often legitimate, but they are not FDA-approved products themselves. The FDA regulates the facility and the process, not each compounded batch the way it approves a brand-name drug.