Polymyxin B vs Thymalin
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
Russian thymic peptide complex that rejuvenates immune function and supports healthy aging
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
Thymalin
5–10 mg
Frequency
Polymyxin B
Multiple times daily
Thymalin
Once daily
Administration
Polymyxin B
Intravenous infusion (on the FDA label)
Thymalin
Intramuscular injection
Cycle length
Polymyxin B
7-14 days, infection-dependent — no source sets a fixed course
Thymalin
4-6 weeks
Onset speed
Polymyxin B
Rapid (hours to days)
Thymalin
Moderate (1-2 weeks)
Evidence level
Polymyxin B
Strong human trials (Phase 3 or FDA approved)
Thymalin
Limited human trials
Benefit ratings
Fighting Resistant Infections
Stopping Bacterial Toxins
Wound Protection
Immune Restoration
Longevity Support
Disease Prevention
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)
Thymalin
Molecular formula
Complex mixture; active dipeptide EW: C16H19N3O5
Molecular weight
Polypeptide complex; components 250-1000 Da; active dipeptide EW ~321 Da
Half-life
Not precisely characterized for complex; immune effects persist weeks post-administration
Bioavailability
High via intramuscular or subcutaneous injection
CAS number
63958-90-7
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.
Thymalin
Intramuscular injection
5-10 mg reconstituted in saline
Once daily · 5-10 day course, often repeated every 6 months
Reflects the Russian clinical course used in immune-restoration and gerontology protocols, typically 10 mg IM daily for 5-10 days [1].
Subcutaneous injection
10 mg
Once daily · Up to 20 day course, repeated every 6 months
Subcutaneous administration is used in community practice at the same 10 mg daily dosing; longer 20-day courses are described for cyclical immune support [1].
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.
Thymalin
Aging Adults Seeking Immune Support
Thymalin is especially valuable for people over 55 whose thymus gland has naturally shrunk with age. By stimulating thymic function, it helps restore the immune training center that your body relies on to fight infections and stay healthy.
Longevity and Anti-Aging Protocols
Clinical studies tracking patients for 6-8 years showed Thymalin reduced mortality by 2.0-2.1-fold. Combined with Epitalon, the mortality reduction reached an impressive 4.1-fold, making this combination one of the most studied longevity peptide protocols.
Seasonal Illness Prevention
Research shows Thymalin cuts respiratory infection rates by 2.0-2.4-fold. A short 10-day course before cold and flu season can significantly boost your immune defenses for months afterward.
Side effects
Polymyxin B
Common
- Nephrotoxicity
- Infusion-related histamine release
- Neurotoxicity
- Skin hyperpigmentation
Uncommon
- Neuromuscular blockade
Serious
- Acute kidney injury requiring dialysis
Thymalin
Common
- No side effects in most patients
- Mild injection site reaction
Uncommon
- Transient low-grade fever
- Mild flu-like symptoms
- Temporary headache
Serious
- Severe 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
Thymalin
Evidence level
Limited human trials
FDA status
Research compound
Safety overview
Thymalin has an excellent safety profile based on over 40 years of clinical use in Russia. Most patients experience zero side effects, and the peptide complex has been described as 'practically non-toxic' in research literature. It has been used safely in thousands of patients, including elderly populations over extended periods.
Contraindications
- Active thymic tumors or malignancies
- Known hypersensitivity to animal-derived peptide products
- Pregnancy and breastfeeding (insufficient safety data)
- Severe uncontrolled autoimmune disease 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 Thymalin if...
- Immune system restoration in aging adults
- Longevity and healthy aging support
- Reducing infection frequency and severity
- Multi-system health optimization