Naltrexone (Standard-dose + Low-Dose / LDN)

Naltrexone is two different drugs at two different doses. At 50 mg it's a full mu-opioid antagonist for alcohol/opioid use disorder (FDA-approved, A-tier evidence) — irrelevant to Dylan. At 1.5–4.5 mg HS ("LDN"), the brief receptor blockade triggers endorphin/enkephalin upregulation rebound + TLR4-mediated microglial anti-inflammatory action — used off-label by a large community for autoimmune disease, fibromyalgia, MS, Crohn's, and post-2020 long COVID. For Dylan (20yo, no autoimmune, no chronic pain) LDN is WATCH-LIST: conceptually interesting for MMA-related neuroinflammation and recovery, but not yet justified by his current symptom picture. Becomes OPTIONAL-ADD if chronic inflammation markers emerge in June bloodwork or if recovery plateaus on V5. Critical opioid-emergency caveat for an MMA athlete: any naltrexone (even LDN) blocks opioid analgesia — surgical, dental, or fracture pain management is fully or partially blunted, and naltrexone in someone using opioids precipitates withdrawal.

Naltrexone is a competitive opioid receptor antagonist with the unusual property that its clinical effects bifurcate completely based on dose. The 50 mg "standard" use case and the 1.5–4.5 mg "LDN" use case are mechanistically distinct enough that the medical literature now treats them as separate interventions sharing only a parent molecule.

1. High-dose (50 mg PO daily, or Vivitrol 380 mg IM monthly) — full mu-opioid blockade. At 50 mg, naltrexone occupies ~90%+ of central mu-opioid receptors for ~24 hours and partially blocks kappa and delta receptors. The clinical effect is complete blunting of opioid reward — both exogenous opioids (heroin, oxycodone, hydrocodone) and the endogenous opioid system response to alcohol consumption are inactivated. This is how it works in alcohol use disorder (AUD): drinking still happens but loses its pleasure signal, breaking the reinforcement loop. In opioid use disorder (OUD) it functions as an antagonist (vs. methadone/buprenorphine which are agonists). Vivitrol is the long-acting IM depot version (380 mg q4wk).

2. Low-dose ("LDN", 1.5–4.5 mg HS) — opioid receptor blockade as a stimulus, not an endpoint. At 1.5–4.5 mg taken at bedtime, naltrexone provides a brief, partial 4-6 hour blockade of opioid receptors. The body responds homeostatically by upregulating production of endogenous opioids — beta-endorphin, met-enkephalin, leu-enkephalin — and by upregulating opioid receptor density. By the time the naltrexone clears (half-life ~4 h, fully gone by morning), the endogenous opioid system is operating at elevated baseline tone. The therapeutic effect comes from this rebound, not from the blockade itself. Bernard Bihari's original 1985 hypothesis was that this "opioid system tonic upregulation" had downstream immunomodulatory effects — which has held up, with caveats, for ~40 years.

3. Low-dose — TLR4 antagonism on microglia (the more recently characterized arm). LDN also acts as a non-opioid antagonist at toll-like receptor 4 (TLR4), particularly on microglia (CNS resident macrophages) and peripheral macrophages. TLR4 is the receptor that detects bacterial LPS (innate immune activation) and is also a key driver of "sterile" neuroinflammation — chronic microglial activation in conditions like fibromyalgia, chronic pain syndromes, long COVID, and post-traumatic / post-impact CNS inflammation. Linda Watkins (Colorado) and Mark Hutchinson's lab characterized the TLR4 antagonism arm in the 2010s; this is now considered the more likely mechanism for LDN's chronic pain and fibromyalgia effects, with the endorphin-rebound mechanism being more relevant to autoimmune effects. For an MMA athlete with subconcussive impact exposure, the TLR4 / microglia angle is the theoretically interesting one — though the human evidence here is still small-RCT level.

4. Glia-modulation downstream effects. By dampening TLR4 signaling on microglia, LDN reduces production of pro-inflammatory cytokines (TNF-alpha, IL-6, IL-1beta) and pro-inflammatory chemokines in CNS. This is the proposed bridge between LDN and central sensitization conditions (fibromyalgia, chronic widespread pain, CFS/ME).

5. Stereochemistry note (for completeness). The (+)-naltrexone enantiomer is TLR4-active but has minimal opioid activity; the (-)-naltrexone enantiomer carries the opioid antagonism. Clinical naltrexone is racemic, so you get both arms. Some research labs now isolate (+)-naltrexone to study TLR4 effects without opioid blockade, but this is not commercially available.

Pharmacokinetics: Oral bioavailability ~5-40% (heavy first-pass — variable by individual). Tmax ~1 hour. Half-life ~4 hours (parent), ~13 hours (active metabolite 6-beta-naltrexol). Hepatic metabolism (primarily UGT, secondary CYP). Renal excretion. At LDN doses, the systemic effect window is roughly 4-6 hours — which is why HS dosing aligns the "blockade window" with the pre-dawn endorphin rebound.