Common (>10% in clinical trials): Dizziness (5-7%), headache (5-6%), confusion (5-6% — primarily in elderly Alzheimer's population), constipation (3-5%). At biohacker doses (5-10 mg/day), all these are <5%.

Less common (1-10%): Insomnia (especially on PM dosing), hallucinations (rare in healthy adults; reported in 2-3% of Alzheimer's trial population, partly disease-related), hypertension (small mean BP rise), agitation, fatigue, somnolence, vomiting, urinary incontinence, anxiety (paradoxical — usually disappears on dose reduction), gait disturbance.

Rare-serious (<1% but worth knowing): Stevens-Johnson syndrome / TEN (extremely rare — one or two case reports across decades; not a mainstream concern). Pancreatitis (case reports). Hepatitis (rare, idiosyncratic). Neuroleptic malignant syndrome-like reactions (extremely rare, mostly when combined with other dopamine-active drugs). Seizure threshold reduction (theoretical; rare in practice). Acute renal failure (extremely rare).

Specific watch periods:

• First 2-4 weeks of titration: dizziness, mild cognitive blunting, sleep disturbance. Most users adapt.
• First 8 weeks at steady state: the unusual subset develops paradoxical agitation or paranoid ideation — discontinue.
• Ongoing: if alkaline urine pH (some bicarbonate-using athletes, vegan diets), watch for accumulation. Monitor renal function annually if used >12 months.

Theoretical concern for healthy young brain (relevant-to-archetype): Chronic NMDA receptor blockade in a still-developing brain. The brain continues maturing into the mid-20s (myelination, prefrontal pruning, executive network refinement). NMDA-receptor signaling drives experience-dependent plasticity throughout this window. Memantine's "physiology-sparing" mechanism is the steel-man counterargument — it preferentially blocks pathological extrasynaptic NMDA signaling, theoretically leaving developmental plasticity intact. Animal and pediatric data (cranial-irradiation neuroprotection trials, autism trials) suggest memantine is well-tolerated developmentally and may even enhance hippocampal neurogenesis. But — there are no long-term studies of chronic memantine in healthy 20-year-olds. The honest verdict: low theoretical concern based on mechanism, no empirical reassurance for healthy young brains specifically. This is one reason the this-archetype verdict is OPTIONAL-WITH-CAVEATS rather than OPTIONAL-ADD.

Cognitive impairment risk specifically: Multiple animal studies (Frankiewicz 1998, Creeley 2006, others) explicitly tested whether neuroprotective concentrations of memantine impair learning or LTP — they don't, at 5-20 mg/kg in rats (which approximates clinical human concentrations). Memantine prevents Aβ-induced LTP impairment and restores LTP in AD models. So the mechanism-level prediction is "no learning impairment at therapeutic doses." Biohacker reports of cognitive blunting at 15-20 mg/day in healthy users suggest the prediction breaks down at the upper end of therapeutic dosing. Stay at 5-10 mg/day for prophylactic / tolerance-modulation use.

No SJS/DRESS-tier dermatologic risk comparable to modafinil. Skin-watch period not required.

No hepatotoxicity signal comparable to bromantane/cerebrolysin. Liver panels not strictly required.

No HPG-axis effects. Memantine does not suppress LH/testosterone. Stack-safe with enclomiphene if the user ever cycles into HPG support.