LDN Fillers: What’s Available, What Works, and What to Avoid
LDN Fillers: What’s Available, What Works, and What to Avoid
A Clinician’s Evidence-Based Guide to Compounded LDN Excipients
Yoon Hang Kim, MD, MPH | Board-Certified in Preventive Medicine | Integrative & Functional Medicine Physician
Published June 2026
Introduction: Why LDN Fillers Matter More Than You Think
Low dose naltrexone (LDN) is typically compounded in doses of 1.5 mg to 4.5 mg—yet the capsule itself can hold approximately 225 mg of powder.1 This means the active pharmaceutical ingredient (API) occupies roughly 2% of the capsule, while the remaining 98% consists of filler, also known as an excipient.1 The choice of filler is not pharmacologically trivial. LDN’s therapeutic mechanism is believed to depend on a rapid blood-level spike followed by a rebound upregulation of endogenous opioid signaling—meaning the formulation must deliver naltrexone as an immediate-release product.2,3 Any filler that slows dissolution, binds to the API, or interferes with gastrointestinal absorption can theoretically undermine efficacy.
Despite this clinical importance, there is no published randomized controlled trial directly comparing LDN fillers head-to-head. The available guidance comes from compounding pharmacy experience, LDN-specific clinical organizations, pharmaceutical science principles, and a modest body of secondary literature. This article synthesizes these sources transparently, with evidence quality noted for each claim.
Core Principle: Immediate Release Is Non-Negotiable
The single most consistent recommendation across all LDN guidance sources is that LDN must be compounded as an immediate-release formulation. The LDN Research Trust’s U.S. physician information packet states: “Pharmacies should be instructed NOT to provide LDN in an ‘SR’ or slow-release or timed-release form. Unless the low dose of naltrexone is in an unaltered form, which permits it to reach a prompt ‘spike’ in the blood stream, its therapeutic effects may be inhibited.”3
LDNscience corroborates this, noting that slow-release or extended-release formulations “should be avoided because they prevent the necessary rebound effect that makes low dose naltrexone effective.”4 This principle extends to topical cream formulations intended for systemic effect, which LDNscience also warns against because transdermal absorption is too slow to produce the required pharmacokinetic spike.4
Evidence quality: Strong expert consensus across multiple independent organizations. Consistent with the pharmacokinetic rationale underlying LDN therapy (transient opioid receptor blockade followed by endorphin rebound). No contradicting sources identified.
Filler-by-Filler Analysis
Avicel® / Microcrystalline Cellulose (MCC)
Avicel (a brand of microcrystalline cellulose manufactured by DuPont) is one of the most widely used pharmaceutical excipients globally and is listed as an inactive ingredient in the FDA-approved label for standard 50 mg naltrexone tablets.5 In the compounding world, MCC is the default filler for many experienced LDN pharmacies. The Compounder, a prominent LDN-focused compounding pharmacy, states: “We use a substance called Avicel (microcrystalline cellulose). It is a good filler. It is inert and it does not form a sticky mass when exposed to water (like methylcellulose).”1 The LDN Research Trust’s U.S. doctor packet explicitly recommends Avicel as a preferred filler.3
However, a notable dissenting view comes from LDNscience, whose Buyer’s Guide states that “cellulose or plant-fiber fillers are more likely to bind with the LDN and slow its absorption” and recommends they be “best avoided.”4 Their Q&A page adds nuance, acknowledging that “microcrystalline cellulose and dextrose are both reported to be compatible with LDN” while still cautioning that cellulose fillers “may be more likely to bind with LDN and slow its absorption, so one may choose to avoid them.”6
Clinical bottom line: Avicel/MCC has the longest track record in LDN compounding and is endorsed by the LDN Research Trust and multiple experienced pharmacies. LDNscience raises a theoretical absorption concern that has not been validated in clinical studies. Thousands of patients use Avicel-filled LDN successfully. It remains a defensible first-line choice, though patients who feel their LDN is not working optimally may consider switching to a sugar- or protein-based filler as a trial.
Calcium Carbonate — Avoid
This is the single most unanimous recommendation in LDN compounding guidance. The LDN Research Trust doctor packet attributes the finding to compounding pharmacist Dr. Skip Lenz, who “demonstrated that the use of calcium carbonate as a filler will interfere with absorption of the LDN capsule,” recommending Avicel, lactose, or sucrose instead.3
The Compounder independently corroborates this: “We don’t use calcium carbonate in our filler because there have been reports that it can interfere with the absorption of naltrexone. If packed too tightly in the capsule it can be difficult to dissolve in the gut. It could then just pass through the body without any effect.”7
LDNscience provides a mechanistic explanation, stating that calcium carbonate “binds to the Naltrexone, turning it into a ‘slow-release’ formulation that prevents the necessary rebound effect that makes LDN effective.”6 Multiple compounding pharmacies echo this, noting that filler choice “is imperative” to avoid “interference with LDN absorption, as seen with calcium carbonate.”8,9
Evidence quality: Unanimous expert consensus across all identified LDN organizations and compounding pharmacy sources. Based on compounding experience and pharmacokinetic reasoning rather than a formal RCT comparing calcium carbonate vs. alternative fillers. The mechanistic plausibility (calcium carbonate forming a poorly soluble matrix under gastric conditions, effectively producing a slow-release formulation) is consistent with pharmaceutical science principles. Patients should also be advised to avoid taking calcium carbonate supplements at the same time as their LDN dose.6
Sucrose (Table Sugar)
Sucrose is endorsed as an acceptable filler by the LDN Research Trust,3 LDNscience,4 and The Compounder.10 LDNscience specifically states that “simple-sugar fillers such as sucrose, dextrose, or lactose do not slow down LDN absorption.”4 As a simple sugar, sucrose dissolves rapidly in gastric fluid and does not bind to naltrexone, making it pharmacokinetically favorable. The amount used as filler (approximately 200 mg) is nutritionally negligible. Sucrose avoids the cellulose debate entirely, making it an appealing option for patients who are uncertain about MCC.
Dextrose (Glucose)
Dextrose shares the same endorsement profile as sucrose—listed by LDNscience as a simple-sugar filler that does not slow absorption.4 It is highly water-soluble and dissolves rapidly. Like sucrose, the ~200 mg quantity is clinically insignificant from a glycemic perspective.
Lactose
Lactose is recommended by the LDN Research Trust as an acceptable filler3 and is confirmed by LDNscience as a simple sugar that does not slow LDN absorption.4 However, it carries an important caveat: LDNscience advises that “lactose should be avoided by individuals with lactose intolerance, gastrointestinal issues, irritable bowel syndrome (IBS), inflammatory bowel disease (IBD, such as Crohn’s and Colitis), and leaky gut syndrome.”4 The Compounder goes further, stating they “strongly advise against lactose (even though some doctors recommend it)” without elaborating on the specific rationale.1
Clinical note: Given that a significant proportion of LDN patients have autoimmune, inflammatory, or GI-related conditions—precisely the populations for whom lactose is cautioned against—lactose is probably best reserved for patients with known tolerance and no GI comorbidity.
Mannitol
Mannitol is a sugar alcohol widely used as a pharmaceutical excipient with good compressibility and flow properties. It is offered as a filler option by CareFirst Specialty Pharmacy for LDN capsules.11 Mannitol is non-hygroscopic, has a pleasant sweet taste, and is compatible with most APIs. It dissolves readily and would not be expected to slow naltrexone absorption. However, at high doses mannitol can have an osmotic laxative effect—though the ~200 mg used as a capsule filler is far below the laxative threshold (typically >10 g). Mannitol may be a good choice for patients avoiding both cellulose and sugars.
Glycine
LDNscience specifically endorses protein-based fillers, stating that “amino acids such as glycine do not slow down LDN absorption.”4 Glycine is highly water-soluble, well-tolerated, and carries its own modest neuromodulatory profile (glycine is an inhibitory neurotransmitter), though the ~200 mg filler dose is far below a therapeutically meaningful quantity. It represents an evidence-supported alternative for patients who wish to avoid both cellulose and sugars.
LoxOral® (PCCA Proprietary Base)
LoxOral is a proprietary all-in-one capsule compounding excipient manufactured by Professional Compounding Centers of America (PCCA). It is designed to improve dissolution time, solubility, and flow characteristics while reducing static in powder blends.12 In June 2025, LoxOral became one of the first compounding excipients to achieve USP Ingredient Verification, providing an independent quality benchmark.13 It is offered as an LDN filler option by some compounding pharmacies. Its proprietary composition means prescribers should verify its ingredients if patients have known excipient sensitivities.
Rice Flour / Rice Powder
Rice powder is mentioned by The Compounder as an alternative for patients who object to MCC, lactose, or methylcellulose: “People who object to microcrystalline cellulose, lactose, or methylcellulose may be happy with rice powder.”14 Rice flour is gluten-free, hypoallergenic, and used broadly as a supplement excipient by major manufacturers.15 However, it is a plant-fiber-derived material, which theoretically places it in the category LDNscience cautions against. There is no published LDN-specific data on rice flour as a filler. It may be a reasonable option for patients with multiple intolerances, but should be considered with the same caveats as other plant-fiber excipients.
Ascorbic Acid (Vitamin C)
Ascorbic acid is offered as a filler option by at least one LDN compounding pharmacy.11 At ~200 mg, it provides a modest antioxidant supplement alongside the LDN dose. However, ascorbic acid is not pharmacologically inert—it is acidic (pH ~2.5 in solution), can affect gastric pH, and has its own biological activity. The Compounder notes they will “object if the filler might be considered another therapeutic item,”10 and vitamin C at 200 mg could arguably fall into that category. There is no published evidence that ascorbic acid interferes with naltrexone absorption, but the principle of filler inertness would favor other options for most patients.
Acidophilus / Probiotic Fillers
Some patients request acidophilus (Lactobacillus) as their LDN filler. The Compounder has addressed this directly and at length, concluding that acidophilus is “technically not an inert filler” because it is a live, active biological substance.14 They raise several practical concerns: most acidophilus products contain dextran produced by corn fermentation (a potential sensitivity trigger); freeze-dried organisms reactivate upon moisture exposure during compounding, raising questions about whether the bacteria could alter the bioavailability or content of the active ingredient; and there is no established documentation of safety for acidophilus as a pharmaceutical excipient.14
Furthermore, The Compounder notes that as a 503A patient-specific compounding pharmacy, they are “not permitted to compound with probiotics (acidophilus included) as it is not something covered by our exemption.”14 While some pharmacies do offer acidophilus as an option when specifically prescribed by a physician, it departs from the principle that fillers should be inert and well-documented.
Sodium Bicarbonate
The Compounder states they would “have no problem making LDN with sodium bicarbonate or even sucrose,” noting that ~200 mg of baking soda is “usually irrelevant to health”—far less than the amount in a typical cupcake.10 However, they note that “in some very rare situations, the added sodium might be an issue.”10 Sodium bicarbonate is not specifically endorsed or cautioned against by the major LDN organizations.
Methylcellulose and Derivatives — Avoid
The Compounder explicitly avoids methylcellulose: “We do not use any derivative of methylcellulose in naltrexone capsules because those substances can make the formulation slow to release.”1 Methylcellulose forms a viscous gel when exposed to water, which can delay dissolution and produce slow-release characteristics—directly contradicting the immediate-release requirement for LDN. This is distinct from microcrystalline cellulose (Avicel), which does not gel in water. MCAS-specific guidance also raises the concern that patients with pine tree pollen sensitivity may react to methylcellulose.16
Liquid, Sublingual, and Alternative Delivery
For patients who are intolerant of multiple fillers—a common scenario in mast cell activation syndrome (MCAS), multiple chemical sensitivity, or severe GI disease—liquid formulations eliminate the filler question entirely. LDNscience describes liquid/drops as offering “fastest absorption” with the ability to make very small dose modifications, though they note some formulations have shorter shelf stability and may require refrigeration.4 Suitable liquid vehicles include propylene glycol (methylethylene glycol) and glycerine.4
The LDN Research Trust notes that liquid formulations can be prepared with minimal preservatives, making them suitable for highly sensitive patients, and recommends refrigerated storage with a shelf life of up to 3 months.3
Summary Table: LDN Fillers at a Glance
Special Considerations: MCAS and Filler Sensitivity
Patients with mast cell activation syndrome deserve special attention regarding filler selection. Dr. Lawrence Afrin, a leading MCAS authority, has written extensively on excipient-directed reactivity, noting that adverse reactions to medications in MCAS patients are frequently caused by fillers, dyes, and coatings rather than the active drug itself.16 He specifically notes that patients with pine tree pollen sensitivity may react to methylcellulose, since it is derived from plant cellulose.16
For MCAS patients, the recommended approach is to start with the simplest possible formulation—ideally a liquid preparation with minimal excipients—and, if a capsule is preferred, to choose a filler with the lowest known reactivity profile. Sugar-based fillers (sucrose, dextrose) or amino acid fillers (glycine) may be better tolerated than cellulose-derived products in this population. As Afrin notes, if a patient reacts to one formulation of a medication, that “almost certainly is an issue of excipient-directed reactivity, not drug-directed reactivity.”16
Key Takeaways for Prescribers
First, always specify immediate-release on the prescription. This is the most important compounding instruction for LDN.
Second, explicitly instruct compounding pharmacies to avoid calcium carbonate and methylcellulose derivatives as fillers.
Third, for most patients, Avicel/MCC or sucrose are well-established, effective first-line filler choices. Avicel has the longest track record; sucrose avoids the cellulose debate entirely.
Fourth, for MCAS patients or patients with multiple chemical sensitivities, consider liquid formulations or capsules filled with glycine, sucrose, or dextrose.
Fifth, if a patient reports that their LDN “stopped working” or “never worked,” inquire about the filler and formulation type before adjusting the dose or abandoning the medication.
Sixth, advise patients not to take calcium carbonate supplements concurrently with their LDN dose.
Limitations and Honest Appraisal of the Evidence
It must be acknowledged that the evidence base for LDN filler selection is composed almost entirely of expert compounding opinion, organizational guidance documents, pharmaceutical science principles, and patient-reported experience. No randomized controlled trial has directly compared LDN fillers. The calcium carbonate avoidance recommendation, while unanimous across sources, traces primarily to the work of a single compounding pharmacist (Dr. Skip Lenz) and has been propagated through LDN advocacy channels. The disagreement between the LDN Research Trust (pro-Avicel) and LDNscience (cautious about cellulose) has not been resolved by comparative data. Prescribers should apply these recommendations with appropriate clinical judgment and individualize filler selection based on patient tolerance and response.
References
1. Frieders L. LDN Fillers. The Compounder. November 15, 2016. Available at: https://thecompounder.com/ldn-fillers/
2. Younger J, Parkitny L, McLain D. The use of low-dose naltrexone (LDN) as a novel anti-inflammatory treatment for chronic pain. Clin Rheumatol. 2014;33(4):451–459.
3. LDN Research Trust. Low-Dose Naltrexone (LDN) Fact Sheet — U.S. Doctor Information Packet. 2014. Available at: https://ldnresearchtrust.org/sites/default/files/Doctors-info-pack-US.pdf
4. LDNscience. The LDN Buyer’s Guide. MedInsight Research Institute. Available at: https://www.ldnscience.org/patients/ldn-buyers-guide
5. DailyMed. Naltrexone Hydrochloride Tablet Label. U.S. National Library of Medicine. Available at: https://dailymed.nlm.nih.gov
6. LDNscience. Q&A: What Medications Can I Take with LDN? Available at: https://www.ldnscience.org/patients/qa/taking-ldn-and-other-substances-treatments
7. Frieders L. LDN Fillers — Calcium Carbonate. The Compounder. November 15, 2016. Available at: https://thecompounder.com/ldn-fillers/
8. Aiken Family Pharmacy. LDN Therapy & Low Dose Naltrexone. Available at: https://www.aikenpharmacy.com/low-dose-naltrexone-ldn
9. Pharmacy Specialists Compounding Pharmacy. Low-Dose Naltrexone. Available at: https://makerx.com/makerx-low-dose-naltrexone
10. Frieders L. Fillers for LDN. The Compounder. June 27, 2017. Available at: https://thecompounder.com/2017/06/27/fillers-for-ldn/
11. CareFirst Specialty Pharmacy. Low Dose Naltrexone (LDN). Available at: https://www.cfspharmacy.pharmacy/human-medicine/low-dose-naltrexone
12. PCCA. LoxOral Product Page. Professional Compounding Centers of America. Available at: https://www.pccarx.com/products/PCCALOXORAL
13. PCCA. PCCA Sets Industry Benchmark as First Supplier with USP Verified Excipient Compounding Bases. GlobeNewswire. June 10, 2025.
14. Frieders L. Acidophilus as LDN Filler? NO. The Compounder. November 10, 2022. Available at: https://thecompounder.com/acidophilus-as-ldn-filler-no/
15. NOW Foods. Supplement Ingredients: Functional Dietary Supplement Ingredients. Available at: https://www.nowfoods.com/now/nowledge/excipients-functional-ingredients-you-may-not-know-about
16. Afrin LB. Approach to Mast Cell Activation Syndrome: Diagnosis and Management. Unpublished clinical guidance document.
17. Patten DK, Schultz BG, Berlau DJ. The Safety and Efficacy of Low-Dose Naltrexone in the Management of Chronic Pain and Inflammation in Multiple Sclerosis, Fibromyalgia, Crohn’s Disease, and Other Chronic Pain Disorders. Pharmacotherapy. 2018;38(3):382–389.
About Dr. Kim
Dr. Yoon Hang “John” Kim is a board-certified physician in Preventive Medicine with over 20 years of clinical experience in integrative and functional medicine. He completed his fellowship under Dr. Andrew Weil at the University of Arizona and holds certifications in preventive medicine, medical acupuncture, and integrative/holistic medicine. Dr. Kim specializes in low dose naltrexone (LDN) therapy, autoimmune conditions, chronic pain, integrative oncology, fibromyalgia, chronic fatigue syndrome, mast cell activation syndrome (MCAS), and mold toxicity. He is the author of three books and more than 20 articles on integrative medicine topics.
Professional: www.yoonhangkim.com |
Clinical: www.directintegrativecare.com
