What's in an Allergy Shot? Safety, Regulation and Ingredient Transparency

No, standard allergen immunotherapy shots do not contain mercury (thimerosal) or any mercury-based preservative. This is a common concern patients raise because they associate injectable medications with thimerosal, which was historically used in some multi-dose vaccine vials. Allergy shot vials use phenol (0.4%) as their bacteriostatic preservative — not thimerosal. Phenol has been used in injectable pharmaceuticals for over a century and has a well-established safety profile at the concentrations used in allergy extracts. AAAAI patient education materials explicitly confirm that allergy shots contain no mercury. If you have concerns about any specific ingredient in your vial, you can ask your allergist for the extract manufacturer's package insert, which lists all components and their concentrations.

Standard allergen immunotherapy shots do not contain aluminum adjuvants. Aluminum is used in some vaccines to enhance immune response, but allergy shots do not require adjuvants because the goal is to induce tolerance rather than a robust immunological attack response. The immune stimulation provided by repeated subcutaneous allergen delivery is sufficient to drive the tolerogenic Th2-to-Treg shift without adjuvant assistance. Alum-precipitated extracts are an older historical formulation that combined allergen extracts with alum to slow absorption, but these are rarely used today and have been largely replaced by aqueous extracts. If you are receiving standard aqueous allergen immunotherapy, no aluminum adjuvants are present.

Yes, allergen extracts used in allergy shots are FDA-regulated biological products. They fall under the authority of the FDA Center for Biologics Evaluation and Research (CBER) — not the Center for Drug Evaluation and Research (CDER) that regulates conventional pharmaceuticals. Manufacturers must produce extracts under current Good Manufacturing Practice (cGMP) conditions and submit each production lot for FDA review before release. FDA-standardized allergen extracts — covering cat hair, dust mites, eight grass species, short ragweed, and Hymenoptera venoms — must demonstrate consistent potency measured in Bioequivalent Allergy Units (BAU) or Allergy Units (AU). The final patient-specific vial compounding performed by allergists follows AAAAI and ACAAI professional guidelines rather than an individual FDA product approval.

Phenol at 0.4% concentration in allergy shots is not dangerous at the amounts used and has a 100+ year safety track record in injectable pharmaceuticals. Phenol is a standard bacteriostatic preservative used to prevent bacterial contamination in multi-dose medication vials. The concentration in allergy extracts (0.4%) is well within limits established as safe for subcutaneous injection by the United States Pharmacopeia (USP). Given that each injection contains only 0.05-0.5mL of solution, the actual amount of phenol delivered per injection is negligible in absolute terms. Phenol sensitivity is extremely rare. If you have concerns about phenol, discuss them with your allergist, who can verify the exact concentration in your specific extract formulation from the manufacturer package insert.

Human serum albumin (HSA) is the most abundant protein in human blood plasma, comprising approximately 60% of total plasma protein in healthy adults. In allergy shots, HSA is used at 0.03% concentration as a protein stabilizer — it prevents allergen extract proteins from adsorbing (sticking) to the glass walls of the vial, which would cause unpredictable reductions in the effective dose. HSA used in pharmaceutical products is derived from carefully screened human plasma donors and processed under FDA-regulated conditions to eliminate the risk of bloodborne pathogen transmission, using techniques similar to those applied to other plasma-derived products like intravenous immunoglobulin (IVIG). Severe albumin allergy is extremely rare and would typically be identified through your medical history review before immunotherapy begins.

FDA-standardized allergen extracts have verified, consistent potency measured in Bioequivalent Allergy Units (BAU) or Allergy Units (AU) and are available for a specific list of allergens: cat hair/pelt, dust mites (D. farinae and D. pteronyssinus), eight grass pollen species, short ragweed, and Hymenoptera (stinging insect) venoms. Standardized extracts must pass FDA potency testing with each lot, ensuring consistency between batches. Non-standardized extracts — covering trees, most weeds (except ragweed), molds, dog, and cockroach — are measured in Protein Nitrogen Units (PNU) or weight-to-volume (w/v) ratios, which are less precise measures of biological potency. Non-standardized extracts can have meaningful lot-to-lot variability. This is a recognized limitation noted in AAAAI practice parameters, and ongoing research efforts aim to standardize more allergen types over time.

Allergy shot vials are compounded by allergists or their trained staff under aseptic conditions following AAAAI and ACAAI compounding guidelines. Aseptic technique — used in pharmacies and compounding facilities across medicine — involves working in a clean environment with sterilized equipment, using sterile components, and employing procedures that minimize contamination risk. Extract stock concentrates arrive from FDA-licensed manufacturers with established sterility testing. When combined into patient-specific vials, the phenol preservative serves as an additional safeguard against bacterial growth in the mixed vial. Vials are labeled with patient-specific information, allergen contents, concentration levels, and expiration date. They are stored at 2-8 degrees Celsius until use and discarded after the 12-month post-compounding expiration date.