Semax, Selank, and Dihexa are the cognitive-domain peptides patients are reading about. Here is what we know, what we don't, and how a physician should think about prescribing them.
The cognitive peptide category is real, the underlying neuroscience is interesting, and the human evidence is thinner than for our standard stack. We will not market what we do not yet prescribe to most patients. We will publish the evaluation framework so you can think about it the way we do.
Day 2 of the July 23–24, 2026 FDA Pharmacy Compounding Advisory Committee meeting includes Semax and Epitalon. We will update this page after the Committee's recommendations.
A synthetic heptapeptide derived from a fragment of adrenocorticotropic hormone (ACTH). It upregulates brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF), and modulates dopamine and serotonin signaling. Russian clinical literature has used Semax in stroke recovery and ADHD-spectrum presentations for decades; Western controlled trials remain limited.
Patient interest signal: attention, mental clarity, post-COVID cognitive complaints, recovery from concussion or ischemic events.
What we tell candidates: the BDNF/NGF mechanism is plausible and the safety profile from observational use is favorable. The Western RCT data we want is not yet published. Off-label, intranasal, evaluated case-by-case under physician oversight.
Status (May 2026): under PCAC review July 24, 2026 (Day 2 of meeting). Not currently on 503A Bulks List. Sourced only through compliant 503A partners when prescribed.
A synthetic peptide analog of tuftsin. Functions as a non-sedating anxiolytic — modulates GABA, serotonin, and BDNF without the cognitive blunting or dependence profile of benzodiazepines. Russian clinical use spans generalized anxiety, social anxiety, and stress-induced cognitive impairment. Intranasal administration.
Patient interest signal: performance anxiety, generalized anxiety not requiring SSRI, focus-under-stress, sleep latency driven by mental noise.
What we tell candidates: Selank is the most interesting member of this triad to us clinically — the mechanism is novel, the abuse-potential profile is clean, and the use case is one many of our patients describe verbatim. Evidence is still primarily Russian-language. Off-label, intranasal, under structured monitoring.
Status (May 2026): not currently on 503A Bulks List. Compounded through licensed 503A only when clinically indicated.
An angiotensin IV analog with reported synaptogenic activity — promotes formation of new synaptic connections in preclinical models, including hippocampal slice preparations. Investigational. The neuroscience is striking on paper; the human data are sparse.
Patient interest signal: early cognitive decline, memory complaints, post-concussive cognition.
What we tell candidates: we are not currently prescribing Dihexa as a standing protocol. The mechanism is interesting; the published human evidence is not yet at the threshold we require. We will continue to follow the literature and re-evaluate.
Status (May 2026): investigational. Limitless does not currently include Dihexa in standing protocols.
1. Is the mechanism biologically plausible and characterized?
BDNF upregulation (Semax), GABA/tuftsin modulation (Selank), and angiotensin IV–pathway synaptogenesis (Dihexa) are all real biology. Plausibility is necessary but not sufficient.
2. Is there usable human safety data?
Decades of Russian observational use for Semax and Selank constitute meaningful — if imperfect — safety signal. Dihexa lacks comparable human data.
3. Is the clinical complaint better-served by an established therapy?
Most cognitive complaints have a workup before they have a peptide. Sleep architecture, thyroid, B12, ferritin, depression screening, sleep-disordered breathing, and hormonal status come first.
4. Can we monitor and stop?
Every protocol has a defined trial window, an objective and subjective response measure, and a pre-specified stop point. If we cannot define those, we do not start.
5. Is the supply chain compliant?
503A-licensed pharmacy with current inspection, documented sourcing, USP-grade chemistry, and sterility data. No exceptions.
Endothelial nitric oxide signaling underlies cerebral perfusion as much as it underlies cardiovascular function — which is why our adult-male cognitive workup recommends daily low-dose tadalafil before most experimental cognitive peptides. Daily tadalafil — one molecule, five systems →
Companion: Peptides — what's real, what's risky, what we prescribe
A Limitless cognitive workup begins with the medical evaluation that should precede any peptide. We screen, we work it up, we treat the obvious causes — and only then, when indicated, do we discuss whether a cognitive peptide protocol is the right next step.
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