Aztreonam (Azactam / Cayston)

How Medicare covers it

Part B in the usual case for the injection (clinician-given; HCPCS J0457 in the CMS Part B spending file — see the Part B basis in this package); the inhaled Cayston form is used at home under Part D. Part D aggregation carried exactly. Part A during a covered inpatient stay.

What it treats

Improving respiratory (breathing) symptoms in people with cystic fibrosis (CF) who have Pseudomonas aeruginosa bacteria in the lungs. It is an inhaled antibiotic, not a general antibiotic for other infections.

What it is for, in depth

Cayston is a monobactam antibacterial indicated to improve respiratory symptoms in cystic fibrosis patients with Pseudomonas aeruginosa. It should be used only to treat CF patients known to have Pseudomonas aeruginosa in the lungs, in order to limit drug-resistant bacteria. It is a chronic, cycling therapy for adults and children 7 years of age and older: 28 days on treatment followed by 28 days off, with the dose not based on weight. Safety and effectiveness have not been established in children under 7, in patients with FEV1 less than 25% or greater than 75% predicted, or in patients colonized with Burkholderia cepacia. Each dose is a single-use vial of sterile lyophilized aztreonam 75 mg plus a 1 mL ampule of sterile diluent, and a 28-day kit contains 84 vials and 88 diluent ampules.

How it works

It is a monobactam antibacterial drug. It blocks the building of the bacterial cell wall by binding to penicillin-binding proteins, which kills susceptible bacteria such as Pseudomonas aeruginosa in the airways.

How it is taken

Breathed in (inhaled) with the Altera Nebulizer System only; it must not be used with any other nebulizer and must not be given by vein or muscle. One single-use vial (75 mg) is mixed with 1 mL of the supplied diluent (0.17% sodium chloride) and used right away, 3 times a day at least 4 hours apart, for a 28-day course, followed by 28 days off treatment. A bronchodilator is used before each dose.

Common side effects

Cough, nasal congestion, wheezing, throat pain (pharyngolaryngeal pain), fever (pyrexia), chest discomfort, abdominal (stomach area) pain, and vomiting. Bronchospasm and rash occur in fewer than 5% of patients.

Serious warnings

Severe allergic reactions have been reported with aztreonam, and an allergic reaction with facial rash, facial swelling, and throat tightness was reported with Cayston in trials; treatment should be stopped if an allergic reaction occurs, and caution is used in people with beta-lactam antibiotic allergy because cross-reactivity may occur. Bronchospasm (airway tightening) can occur with nebulized treatment; a drop of 15% or more in FEV1 right after a dose was seen in 3% of patients. Using it in CF patients without known Pseudomonas aeruginosa infection increases the risk of drug-resistant bacteria without benefit.

Who should be careful

People with a known allergy to aztreonam must not use it. People with a history of allergy to beta-lactam antibiotics (such as penicillins, cephalosporins, or carbapenems) should be treated with caution. Safety and effectiveness have not been established in children under 7 years old, in patients with FEV1 below 25% or above 75% predicted, or in patients colonized with Burkholderia cepacia.

Major interactions

No formal clinical studies of drug interactions with Cayston have been conducted, and the label lists no specific drug interactions. It should not be mixed with any other drugs in the nebulizer handset.

Cost and saving notes

If this drug is covered under your Medicare Part D plan and the cost is high, you can ask your plan for a tier exception to pay a lower cost share, and you can ask your doctor to help with an appeal if the plan says no. If you have limited income, Extra Help (the Part D Low-Income Subsidy) can lower premiums and drug costs, and you can apply through Social Security. In 2027, what you pay out of pocket for Part D drugs is capped at $2,400 for the year.

Why it is in its tier

Aztreonam (Azactam / Cayston) sits on Tier 4 in 79.22% of the plans that list it (4353 of 5495 listing plans). Tier 4 is the non-preferred tier — higher copays or coinsurance. The data-visible drivers: It is sold both as a brand product and in generic form, and the CMS dashboard puts its average Part D spending at about $1,676 per beneficiary for 2024 — a mid-cost drug, which matches where plans place it. It is listed by 100.0% of Part D plans. The tier shown is the most common one; individual plans vary.

Sources

• CAYSTON- aztreonam kit — DailyMed
• DailyMed — Cayston Patient Information (FDA-approved patient labeling in the product label, Gilead Sciences)

What official patient guides say

Cayston is the form of this medicine you use at home: a powder in a single-use vial that you mix with the saline ampule in the kit and breathe in as a mist. Use only the Altera Nebulizer System — no other nebulizer — and do not mix Cayston with any other medicine in it. Mix one vial with one saline ampule only when you are ready for a dose, and use it right away; each treatment takes about 2 to 3 minutes. The usual pattern is 3 doses a day, at least 4 hours apart, for 28 days, then 28 days off, and you should finish the full 28-day course even if you feel better. Use an inhaled bronchodilator before each dose, and if you use several breathing treatments, take them in this order: bronchodilator first, then mucus-clearing medicines, then Cayston last. If you miss a dose, you can still fit in all 3 doses that day as long as they stay at least 4 hours apart. Keep the vials and saline together in the refrigerator and away from light; once they come out, they can stay at room temperature for up to 28 days, and should not be used after that or after the expiration date. (The injection form of aztreonam is given in a clinic or hospital, not at home.)

Source: DailyMed — Cayston Patient Information (FDA-approved patient labeling in the product label, Gilead Sciences)

Part D formulary coverage

Source: CMS Prescription Drug Plan Formulary, Pharmacy Network, and Pricing Information Files — Monthly file 2026-10-15 (plan year 2027), file 2027_20261008.zip (data.cms.gov dataset ed1496ea-23e7-41f1-9292-172fa842ccfb): basic drugs formulary file + plan information file, both dated 20261031 inside the zip; downloaded and aggregated 2026-10-30. Medication RxCUI sets resolved through the NLM RxNav (RxNorm) API.

What you pay

A tier is not a price; what you pay depends on your plan, pharmacy, and benefit stage; Extra Help or Medicaid can lower it further.

Sources: CMS Plan Benefit Package + formulary data; CMS NADAC · Checked 2026-11-07

Read next

• Part D in 2027: The $700 Deductible, the $2,400 Cap, and the $35 Insulin Rule
• Extra Help in 2027: Lower Drug Copays and Who Qualifies
• Checking Medicare Savings Programs and Extra Help

Part D spending averages (2024)

Source: CMS Medicare Part D Spending by Drug (Drug Spending Dashboard), release RY26 P04 V10, data through 2024 (DSD_PTD_RY26_P04_V10_DY24_BGM.csv), downloaded 2026-10-30

Companies covering this drug

5,495 plans from 509 companies list this drug on a formulary for 2027. Search by company or plan name, or filter by tier. Cost sharing is the plan's published amount for this drug's tier in the initial coverage stage (1-month retail supply), where the plan publishes it; a blank means the plan does not publish it.

A tier is not a price; what you pay depends on your plan, pharmacy, and benefit stage; Extra Help or Medicaid can lower it further.