LSD

The original psychoplastogen — a Schedule I 5-HT2A partial agonist with strong A-tier macrodose evidence for treatment-resistant depression, end-of-life anxiety, and alcohol use disorder, but a microdose RCT literature that has largely collapsed under placebo control. For a 20yo with no clinical indication: SKIP-FOR-NOW. The legal exposure (Schedule I, federal felony), still-maturing prefrontal cortex + serotonergic system, HPPD risk, valvulopathy theoretical concern at chronic micro dosing (5-HT2B partial agonism — same receptor that retired fenfluramine), psychosis-precipitation risk in family-history-positive individuals, and dangerous lithium interaction (seizures) make this an easy decline. Documented here for completeness, not advocacy.

LSD is the prototypical classical psychedelic — Albert Hofmann synthesized it from ergot alkaloids in 1938, accidentally dosed himself in 1943, and the resulting bicycle ride is the founding myth of psychopharmacology. Mechanistically it is one of the most receptor-promiscuous compounds in the psychedelic class, with three legs that all matter:

1. The 5-HT2A leg (the primary psychedelic substrate):

• Partial agonist at 5-HT2A, Ki ~1.1 nM. This is the receptor that all classical psychedelics (psilocybin/psilocin, DMT, mescaline, 5-MeO-DMT, 25I-NBOMe, LSD) converge on as the necessary substrate for the subjective psychedelic experience.
• β-arrestin-biased agonism at 5-HT2A — LSD recruits β-arrestin signaling more efficiently than G-protein signaling at this receptor, which appears to be the molecular signature of its psychedelic vs non-psychedelic effects (Wacker et al. 2017; Kim et al. 2020 cryo-EM structures).
• 5-HT2A activation in cortical layer V pyramidal neurons → glutamate release → enhanced cortico-cortical and thalamocortical signaling → the cognitive/perceptual changes that characterize the trip.
• Pre-treatment with a 5-HT2A antagonist (ketanserin) abolishes the subjective psychedelic experience in human studies — this is the cleanest mechanistic proof that 5-HT2A is THE receptor for the trip itself.

2. The psychoplastogen leg (the BDNF/synaptogenesis story):

• Ly et al. 2018 (Olson lab, UC Davis), Cell Reports — landmark paper coining the term "psychoplastogen." LSD (and DMT, psilocin) increases dendritic spine density, dendritic arbor complexity, and synaptic protein levels in cortical neurons in vitro and in vivo. Magnitude is comparable to ketamine. Effects are TrkB/BDNF-dependent and downstream of mTOR signaling.
• The psychoplastogen effect is 5-HT2A-dependent in most assays (ketanserin blocks it), creating the awkward unity of "the trip and the neuroplasticity ride together" — which is exactly what Olson lab is now trying to break apart with non-hallucinogenic 5-HT2A agonists (tabernanthalog, 2-Br-LSD, etc.).
• Single-dose macrodose effects on dendritic structure persist for weeks to months in rodent models — the "afterglow" mechanism that may underlie the rapid + sustained antidepressant effect seen in clinical trials.
• At microdoses, the psychoplastogen effect is uncertain. Some rodent work suggests sub-perceptual doses still drive dendritogenesis (Cameron et al. 2019, ACS Chem Neurosci), but human translation is the open question that the contested microdose RCT literature is trying to answer.

3. The ancillary receptor cocktail (why LSD is not "just psilocybin with a longer trip"):

• 5-HT1A partial agonist — anxiolytic + serotonergic autoreceptor effects, contributes to the calmer subjective profile vs psilocybin in some users
• 5-HT2C partial agonist — modulates appetite, mood, anxiolysis
• 5-HT2B partial agonist — this is the cardiac valvulopathy receptor. Same receptor target as fenfluramine (withdrawn 1997 for valvular heart disease), pergolide (withdrawn 2007 for valvulopathy), cabergoline (still used at low doses for hyperprolactinemia, valvular risk at high Parkinsonism doses). LSD's affinity at 5-HT2B is meaningful, and chronic microdosing (every 3 days for months) raises a legitimate theoretical concern — see Side Effects.
• D2 partial agonist + D1 weak agonist — dopaminergic activity is one of the features that distinguishes LSD from pure 5-HT2A agonists like psilocin. Contributes to the longer subjective duration and the "energetic" character vs psilocybin's "earthier" profile.
• Trace amine receptor TAAR1 agonism + weak action at α1, α2, β adrenergic receptors

4. Default-mode network (DMN) effects (the network-level fMRI story):

• Carhart-Harris and the Imperial College London group (2016 PNAS on LSD fMRI; 2020 review) showed LSD decouples the default-mode network — the resting-state network associated with self-referential processing, autobiographical memory, and "ego" — and increases between-network connectivity (whole-brain entropy increases).
• This is the proposed neural correlate of "ego dissolution," "expanded consciousness," "psychedelic insight," and theoretically the antidepressant mechanism (depression is associated with DMN over-connectivity / rumination; LSD breaks that pattern).
• DMN disruption scales with dose. Macrodoses produce dramatic DMN dissolution; microdoses produce, at most, subtle attenuation that may or may not be clinically relevant.

5. Pharmacokinetics — relevant to dosing and tolerance:

• Oral bioavailability ~70-80% (sublingual blotter standard)
• Onset ~30-60 min, peak 2-4 hr, duration 8-12 hr (longer than psilocybin's 4-6 hr; longer than DMT's 15 min)
• Plasma half-life ~3-5 hr; biphasic elimination
• Hepatic metabolism via CYP enzymes (less well-characterized than psilocybin; CYP2D6 and CYP1A2 implicated)
• Acute tolerance is dramatic — within 1-3 days of repeated dosing, subjective effects flatten almost completely (tachyphylaxis; rapid 5-HT2A receptor downregulation). This is the pharmacological reason macrodose use must be spaced ≥1 week.
• Microdose protocols (every 3 days) attempt to thread the needle between cumulative effect and tachyphylaxis — but the receptor pharmacology suggests measurable downregulation begins within 24-72 hours, which is part of why the microdose pharmacological story is harder to make than the macrodose story.