Common (>10% in biohacker reports / >5% in trials):

• Mild GI upset / nausea — the most common acute effect, usually dose-dependent, mitigated by taking with food.
• Headache — often related to choline depletion (cholinergic enhancement), mitigated by co-administered citicoline/alpha-GPC.
• Fatigue / mild sedation — uncommon at 5-30 mg, more common >100 mg.

Less common (1-10%):

• Dizziness / lightheadedness — usually transient, first 1-3 days.
• Mood shifts — both directions reported (mild irritability or mild euphoria) — variable, probably reflects individual response.
• Sleep disruption — rare at AM dosing; more common with PM dosing or BID late-day dosing (mGluR/cholinergic enhancement can be alerting).

Rare-serious (<1% but worth knowing):

• GABA-B rebound withdrawal on discontinuation after chronic dosing. Pattern: anxiety, irritability, insomnia, in severe cases hallucinations/psychosis (the published phenibut+fasoracetam case report describes day-2 hallucinations and delusions of grandeur on resolution of acute intoxication). Risk increases with duration (>4 weeks daily) and dose (>50 mg/day). Not benign — taper, do not stop cold after extended use.
• Phenibut interaction (case report, severe). Owen et al. Journal of Critical Care Medicine 2018: 27-year-old male on phenibut 500 mg TID + fasoracetam 50-100 mg/day for two days, ingested 10 g phenibut + unknown fasoracetam at a party (empty 5 g fasoracetam bottle found). Presented with GCS 3, sinus bradycardia HR 36 bpm requiring atropine + transcutaneous pacing, intermittent agitation/vomiting. Standard tox screen negative. Day 2: hallucinations + delusions of grandeur on emergence. Mechanism: fasoracetam GABA-B upregulation amplifies phenibut GABA-B agonism, producing additive sedation, novel cardiac toxicity (bradycardia not reported in phenibut-only cases), and severe GABA-B rebound. This combination is not safe at any dose and has a documented near-fatal case.
• Theoretical: aggregation of glutamate-system modulator effects with concurrent TAK-653, neboglamine, memantine, agmatine, or other glutamate-modulators. No case reports, but mechanism makes simultaneous use a multiplied-unknown.
• Long-term safety unknown. Chronic mGluR + GABA-B upregulation has no human safety data beyond ~6 weeks of consistent dosing in controlled trials. Receptor adaptation is documented in rats but human chronic data is absent.

Specific watch periods:

• Days 1-7: GI tolerability, headache (choline status), acute anxiolytic onset.
• Weeks 2-4: mood drift watch, sleep changes, response plateau (does the effect deepen or fade?).
• Discontinuation week 1-2: rebound anxiety, insomnia, irritability. Taper if used >2 weeks consecutively.