Polymyxin B vs Thymosin Alpha-1
Cyclic lipopeptide antibiotic from Paenibacillus polymyxa containing 10 amino acids with 6 diaminobutyric acid residues and a fatty acid tail — FDA-approved since 1964 as a last-resort treatment for multidrug-resistant Gram-negative infections including Pseudomonas aeruginosa, Acinetobacter baumannii, and carbapenem-resistant Enterobacteriaceae, targeting lipid A of bacterial lipopolysaccharide with rapid bactericidal membrane disruption
Your immune system's master trainer—a naturally occurring thymus peptide that wakes up tired immune cells, helps your body fight infections, and keeps your defenses sharp as you age.
Written by Michael Carroll — Owner, Director of Research · Reviewed by the Peptide Initiative Research Team · Editorial standards
Quick comparison
Dose range
Polymyxin B
1.25–1.5 mg/kg
Thymosin Alpha-1
1.6–1.6 mg
Frequency
Polymyxin B
Multiple times daily
Thymosin Alpha-1
Twice weekly
Administration
Polymyxin B
Intravenous infusion (on the FDA label)
Thymosin Alpha-1
Subcutaneous injection
Cycle length
Polymyxin B
7-14 days, infection-dependent — no source sets a fixed course
Thymosin Alpha-1
8-12 weeks
Onset speed
Polymyxin B
Rapid (hours to days)
Thymosin Alpha-1
Moderate (1-2 weeks)
Evidence level
Polymyxin B
Strong human trials (Phase 3 or FDA approved)
Thymosin Alpha-1
Moderate human trials (Phase 1-2)
Benefit ratings
Fighting Resistant Infections
Stopping Bacterial Toxins
Wound Protection
Immune Activation
Infection Fighting
Immune Balance
Compound specifications
Polymyxin B
Molecular formula
C₅₆H₉₈N₁₆O₁₃ (polymyxin B₁ free base)
Molecular weight
1,203.5 g/mol (free base); ~1,385 g/mol (sulfate salt)
Half-life
Terminal half-life: 9-11.5 hours in patients with normal renal function; does not require renal dose adjustment (unlike colistimethate); achieves steady-state within 1-2 days with loading dose
Bioavailability
IV: 100% (direct administration); oral: negligible (not absorbed from GI tract — used topically in the gut for selective decontamination); inhaled: local pulmonary concentrations achieved with systemic absorption variable; topical: minimal systemic absorption
CAS number
1405-20-5 (polymyxin B sulfate)
Thymosin Alpha-1
Molecular formula
C129H215N33O55
Molecular weight
3108.32 g/mol
Half-life
Approximately 2 hours
Bioavailability
High when injected subcutaneously (rapid absorption, peak at ~2 hours)
CAS number
62304-98-7
Dosing compared
Polymyxin B
Intravenous
Loading dose 2.0-2.5 mg/kg (20,000-25,000 units/kg)
Once on day 1 · Day 1 loading, then maintenance
The consensus loading dose for serious multidrug-resistant Gram-negative infection, by total body weight, infused over 1 hour [7]. It is not on the FDA label, which predates this guidance [6]. 1 mg of polymyxin B is 10,000 units — a dose quoted without saying which convention it uses cannot be interpreted [7].
1.25-1.5 mg/kg (12,500-15,000 units/kg) per dose
Every 12 hours · 7-14 days, infection-dependent
Consensus maintenance dosing, infused over 1 hour [7]. That works out to 2.5-3 mg/kg/day, or 25,000-30,000 units/kg/day — ABOVE the FDA label's stated maximum of 25,000 units/kg/day [6], which is one of the outdated-product-information problems the consensus panel was convened to resolve [7]. Unlike colistin, polymyxin B is not dose-reduced for renal impairment or dialysis [7]; the FDA label, written before that was understood, does instruct reduction in renal impairment [6]. This is a hospital drug given by clinicians who know which convention they are using.
Intramuscular
25,000-30,000 units/kg/day divided every 4-6 hours
Every 4-6 hours · Infection-dependent
FDA label intramuscular dosing; not generally recommended due to injection-site pain. [6]
Intrathecal
50,000 units once daily for 3-4 days, then 50,000 units every other day
Daily then every other day · At least 2 weeks after CSF cultures turn negative
FDA label intrathecal regimen for meningitis (adults and children >2 yr); typically given with concomitant IV polymyxin. Children <2 yr: 20,000 units/day for 3-4 days. [6]
Topical
10,000-25,000 units/mL solution, 1-3 drops
Hourly at first, lengthening the interval as the response allows · Until infection resolves
Topical and subconjunctival use for eye infections caused by susceptible Pseudomonas aeruginosa is on the FDA label [6]. The concentration is 0.1-0.25%, which is the same thing as 10,000-25,000 units per mL — written here in units because that is how the rest of this drug's dosing is expressed.
Thymosin Alpha-1
Subcutaneous injection
1.6 mg
Twice weekly (3-4 days apart) · 6-12 months
Established clinical dose for thymalfasin (Zadaxin); body-surface-scaled as 900 mcg/m2 [4][6]. Phase III chronic hepatitis B trials used 1.6 mg SC twice weekly for 6-12 months [4].
1.6 mg daily, then 1.6 mg twice weekly
Daily for an initial loading period, then twice weekly · 12+ weeks
Some immune-adjuvant and acute protocols front-load with daily dosing before settling to the standard twice-weekly schedule [4][6].
Best suited for
Polymyxin B
Treatment of life-threatening multidrug-resistant Gram-negative infections when carbapenems and other agents have failed
Polymyxin B is particularly well-suited for individuals focused on treatment of life-threatening multidrug-resistant gram-negative infections when carbapenems and other agents have failed. Research and clinical experience suggest meaningful benefits in this area when used as part of a comprehensive treatment approach.
Salvage therapy for carbapenem-resistant Acinetobacter baumannii (CRAB) bloodstream infections and pneumonia
Polymyxin B is particularly well-suited for individuals focused on salvage therapy for carbapenem-resistant acinetobacter baumannii (crab) bloodstream infections and pneumonia. Research and clinical experience suggest meaningful benefits in this area when used as part of a comprehensive treatment approach.
Intrathecal/intraventricular treatment of MDR Gram-negative meningitis and ventriculitis
Polymyxin B is particularly well-suited for individuals focused on intrathecal/intraventricular treatment of mdr gram-negative meningitis and ventriculitis. Research and clinical experience suggest meaningful benefits in this area when used as part of a comprehensive treatment approach.
Inhaled therapy for MDR Gram-negative ventilator-associated pneumonia (VAP) as adjunct to systemic therapy
Polymyxin B is particularly well-suited for individuals focused on inhaled therapy for mdr gram-negative ventilator-associated pneumonia (vap) as adjunct to systemic therapy. Research and clinical experience suggest meaningful benefits in this area when used as part of a comprehensive treatment approach.
Thymosin Alpha-1
Chronic Hepatitis B or C Support
Thymosin Alpha-1 has its strongest clinical evidence here. Multiple trials show it helps clear viral loads and normalize liver enzymes, especially when combined with antiviral medications. It's approved in over 30 countries specifically for hepatitis treatment.
Cancer Treatment Support
When used alongside chemotherapy, Thymosin Alpha-1 may help maintain immune function that chemo tends to suppress. Research in lung cancer, melanoma, and liver cancer shows improved outcomes when added to standard treatments. It helps your immune system keep fighting even during tough treatments.
Age-Related Immune Decline
As you age, your thymus shrinks and produces less thymosin naturally—a process called immunosenescence. Supplementing with Thymosin Alpha-1 may help compensate, keeping your immune defenses more youthful and responsive. Think of it as replacing what your body makes less of over time.
Severe Infection Recovery
In sepsis and critical infections, Thymosin Alpha-1 has shown promise for reducing mortality by helping restore immune balance. It's particularly interesting because it modulates immunity rather than just boosting it—calming overreaction while enhancing pathogen-fighting ability.
Side effects
Polymyxin B
Common
- Nephrotoxicity
- Infusion-related histamine release
- Neurotoxicity
- Skin hyperpigmentation
Uncommon
- Neuromuscular blockade
Serious
- Acute kidney injury requiring dialysis
Thymosin Alpha-1
Common
- Injection site reactions
- Mild fatigue
- Flu-like symptoms
Uncommon
- Mild fever
- Lymph node awareness
Serious
- Allergic reaction
Safety & evidence
Polymyxin B
Evidence level
Strong human trials (Phase 3 or FDA approved)
FDA status
FDA approved. Indicated for acute infections caused by susceptible Pseudomonas aeruginosa (urinary tract, meninges, bloodstream), and for serious infections caused by H. influenzae, E. coli, Aerobacter aerogenes and Klebsiella pneumoniae when less toxic drugs are ineffective or contraindicated. Carries a BOXED WARNING. Meningeal infections must be treated by the intrathecal route only.
Safety overview
Polymyxin B carries a BOXED WARNING, and it is the most important thing on this page [6]. When given intramuscularly or intrathecally it is to be given only to hospitalised patients under constant physician supervision. Renal function must be determined before use and dosage reduced in renal damage; nephrotoxicity shows as albuminuria, cellular casts and azotemia, and falling urine output with a rising BUN is an instruction to stop the drug. Neurotoxic reactions present as irritability, weakness, drowsiness, ataxia, perioral paraesthesia, numbness of the extremities and blurred vision, usually with the high serum levels seen in renal impairment. Most seriously, the label states the neurotoxicity CAN RESULT IN RESPIRATORY PARALYSIS FROM NEUROMUSCULAR BLOCKADE, especially when given soon after anaesthesia or muscle relaxants. The boxed warning names the drugs to avoid concurrently or sequentially: bacitracin, streptomycin, neomycin, kanamycin, gentamicin, tobramycin, amikacin, cephaloridine, paromomycin, viomycin and colistin. Safety in human pregnancy has not been established. Beyond the box, the label reports drug fever, urticarial rash, severe pain at intramuscular injection sites, thrombophlebitis at intravenous sites, and Clostridioides difficile associated diarrhoea ranging from mild to fatal colitis, which can begin more than two months after the antibiotic was given [6].
Contraindications
- Prior hypersensitivity reaction to polymyxins — the FDA label's only stated contraindication
- Concurrent or sequential use of other neurotoxic or nephrotoxic drugs, which the boxed warning names: bacitracin, streptomycin, neomycin, kanamycin, gentamicin, tobramycin, amikacin, cephaloridine, paromomycin, viomycin and colistin
- Soon after anaesthesia or muscle relaxants — the boxed warning flags respiratory paralysis from neuromuscular blockade
- Pregnancy — the boxed warning states safety in human pregnancy has not been established
- Myasthenia gravis — neuromuscular blockade is the mechanism behind the label's respiratory-paralysis warning
Thymosin Alpha-1
Evidence level
Moderate human trials (Phase 1-2)
FDA status
FDA approved for other use
Safety overview
Thymosin Alpha-1 has one of the most extensive safety records of any peptide, with decades of clinical use across multiple countries. Studies consistently report minimal side effects—mostly limited to mild injection site reactions. The 2-hour half-life means it doesn't accumulate in your system. It's been used safely in thousands of patients with hepatitis, cancer, and other serious conditions.
Contraindications
- Organ transplant recipients on immunosuppressants
- Active autoimmune disease flares
- Known allergy to thymosin peptides
- Pregnancy or breastfeeding
- Children under 18 without medical supervision
Which is right for you?
Choose Polymyxin B if...
- Treatment of life-threatening multidrug-resistant Gram-negative infections when carbapenems and other agents have failed
- Salvage therapy for carbapenem-resistant Acinetobacter baumannii (CRAB) bloodstream infections and pneumonia
- Intrathecal/intraventricular treatment of MDR Gram-negative meningitis and ventriculitis
- Inhaled therapy for MDR Gram-negative ventilator-associated pneumonia (VAP) as adjunct to systemic therapy
Choose Thymosin Alpha-1 if...
- Immune system strengthening
- Chronic infection support
- Cancer adjunct therapy
- Healthy aging immune support