Peptides Studied for Cognition: What the Evidence Shows

"Nootropic peptides" is a popular label, but the research behind it varies enormously. Some compounds have small human trials, some only rodent data, and one has a founding paper under a formal expression of concern.

This guide covers the peptides most often discussed for memory and focus: what each is, its strongest level of evidence, and one or two PubMed-sourced findings. The goal is to help you read the claims, not to recommend anything.

How to read the evidence levels

We use the research labels from our methodology page:

• Human trials means people were given the compound in a controlled study. That still varies in quality: a randomized controlled trial (RCT) with a placebo group is much stronger than an open comparison.
• Preclinical means cell or animal work only. Results in mice often fail to carry over to people.
• A meta-analysis pools several trials, and is only as good as they are.

Semax

What it is. A synthetic seven-amino-acid peptide built on a fragment of adrenocorticotropic hormone (ACTH 4-10), usually given as a nasal spray (intranasal). It is mainly studied in Russia. See the Semax page.

Strongest evidence: small human studies. In a Russian study of 110 people recovering from ischemic stroke, the groups given Semax had higher blood levels of BDNF (brain-derived neurotrophic factor, a protein involved in nerve-cell growth and plasticity) and better scores on the Barthel index, a measure of daily functioning (Gusev 2018). In 24 healthy volunteers, a single intranasal dose changed resting-state brain-network activity on fMRI compared with placebo (Lebedeva 2018). That second study measured brain imaging, not memory or thinking performance.

Mechanism data. In rats, intranasal Semax raised BDNF protein in the basal forebrain within three hours, and the researchers found specific binding sites for it in that region (Dolotov 2006).

The gap. We found no large, placebo-controlled trial of Semax for cognition in healthy people.

Selank

What it is. A synthetic seven-amino-acid peptide based on tuftsin, a naturally occurring short peptide. It is studied mainly as an anti-anxiety agent, with cognition as a secondary interest. See the Selank page.

Strongest evidence: small human trials against other drugs. In 62 patients with generalized anxiety disorder or neurasthenia, Selank's anti-anxiety effect was similar to that of medazepam, a benzodiazepine (Zozulia 2008). A 60-patient study comparing it with phenazepam reported anti-anxiety and "mild nootropic" effects (Medvedev 2014). Neither abstract describes a placebo group, and both were published in Russian.

Animal data. In rats given alcohol for 30 weeks, Selank prevented memory and attention problems during withdrawal in an object-recognition test (Kolik 2019).

See also Semax vs Selank and the Semax + Selank stack.

N-acetyl and amidated variants

What they are. N-Acetyl Semax, N-Acetyl Selank and their "amidate" versions (N-Acetyl Semax Amidate, N-Acetyl Selank Amidate) are the parent peptides with chemical caps added to one or both ends. Acetylation and amidation are commonly used to slow breakdown by enzymes.

Strongest evidence: chemistry and cell studies. Acetylation is not a neutral change. One lab study found that acetylated Semax binds copper differently from the original, and unlike the original it did not protect neuroblastoma cells from copper toxicity (Magrì 2016).

The gap. We found no human trials of the acetylated or amidated forms. Claims that they are "stronger" or "longer-lasting" versions of Semax and Selank in people are extrapolations, not trial results.

Dihexa

What it is. A small modified peptide derived from angiotensin IV, designed to be taken by mouth and to cross into the brain. See the Dihexa page.

Strongest evidence: animal studies only. The 2013 paper that introduced Dihexa reported that it reversed memory deficits in rat models and increased the formation of new synapses (McCoy 2013). That paper carries an expression of concern, a formal notice from the journal that questions have been raised about the work. The notice was issued in 2021 and is linked from the paper's PubMed record. A related 2014 paper from the same group, which proposed that these effects work through the HGF/c-Met growth-factor system, was retracted in 2025 (Retraction notice 2025).

Independent studies are mixed. A separate group reported that Dihexa improved water-maze performance and reduced brain inflammation markers in APP/PS1 mice, a model of Alzheimer's disease (Sun 2021). Another independent group found it did not protect rats from motor and memory deficits in a model of Huntington's disease (Wells 2024).

The gap. We found no published human trials.

Cerebrolysin

What it is. Not a single peptide but a mixture derived from pig brain, given as a series of intravenous infusions. See the Cerebrolysin page.

Strongest evidence: meta-analyses of human trials, with caveats. A Cochrane review of six trials (597 participants) in vascular dementia found small benefits on cognition and global function but rated the evidence "very low-quality." The authors noted a high risk of bias and that the effects may be too small to be clinically meaningful (Cui 2019). A meta-analysis of six placebo-controlled trials in mild-to-moderate Alzheimer's disease found a cognitive benefit at 4 weeks that was not statistically significant at 6 months, alongside better global clinical ratings (Gauthier 2015).

Context. Cerebrolysin has the most human data here, all in people with diagnosed disease, not healthy adults.

P21 (P021)

What it is. A small synthetic peptide derived from ciliary neurotrophic factor and modified with an adamantane group. See the P21 page.

Strongest evidence: animal studies only. In normal adult mice, P21 improved learning and memory and increased the birth and maturation of new neurons in the hippocampus (Li 2010). In aged rats (22 to 24 months), oral P021 reduced age-related decline in learning and memory (Bolognin 2014). Both studies come from the same research institute.

The gap. We found no human trials.

Summary table

The bottom line

None of these compounds has strong, independently replicated evidence for improving cognition in healthy people. The human data that exist are small, often not placebo-controlled, or in people with stroke or dementia. For Dihexa, the most-cited evidence is now formally in question.

Each peptide page lists its research status and PubMed-verified studies, and you can ask follow-up questions in chat.

Sources

• [The efficacy of semax in the tretament of patients at different stages of ischemic stroke]. Zh Nevrol Psikhiatr Im S S Korsakova, 2018. PMID 29798983
• Effects of Semax on the Default Mode Network of the Brain. Bull Exp Biol Med, 2018. PMID 30225715
• Semax, an analogue of adrenocorticotropin (4-10), binds specifically and increases levels of brain-derived neurotrophic factor protein in rat basal forebrain. J Neurochem, 2006. PMID 16635254
• [Efficacy and possible mechanisms of action of a new peptide anxiolytic selank in the therapy of generalized anxiety disorders and neurasthenia]. Zh Nevrol Psikhiatr Im S S Korsakova, 2008. PMID 18454096
• [A comparison of the anxiolytic effect and tolerability of selank and phenazepam in the treatment of anxiety disorders]. Zh Nevrol Psikhiatr Im S S Korsakova, 2014. PMID 25176261
• Selank, Peptide Analogue of Tuftsin, Protects Against Ethanol-Induced Memory Impairment by Regulating of BDNF Content in the Hippocampus and Prefrontal Cortex in Rats. Bull Exp Biol Med, 2019. PMID 31625062
• Influence of the N-terminus acetylation of Semax, a synthetic analog of ACTH(4-10), on copper(II) and zinc(II) coordination and biological properties. J Inorg Biochem, 2016. PMID 27586814
• Evaluation of metabolically stabilized angiotensin IV analogs as procognitive/antidementia agents. J Pharmacol Exp Ther, 2013. PMID 23055539
• Retraction notice to "The Procognitive and Synaptogenic Effects of Angiotensin IV-Derived Peptides Are Dependent on Activation of the Hepatocyte Growth Factor/c-Met System" [J Pharmacol Exp Ther 351 (2014) 390-402]. J Pharmacol Exp Ther, 2025. PMID 40312093
• AngIV-Analog Dihexa Rescues Cognitive Impairment and Recovers Memory in the APP/PS1 Mouse via the PI3K/AKT Signaling Pathway. Brain Sci, 2021. PMID 34827486
• Effects of an Angiotensin IV Analog on 3-Nitropropionic Acid-Induced Huntington's Disease-Like Symptoms in Rats. J Huntingtons Dis, 2024. PMID 38489193
• Cerebrolysin for vascular dementia. Cochrane Database Syst Rev, 2019. PMID 31710397
• Cerebrolysin in mild-to-moderate Alzheimer's disease: a meta-analysis of randomized controlled clinical trials. Dement Geriatr Cogn Disord, 2015. PMID 25832905
• Neurotrophic peptides incorporating adamantane improve learning and memory, promote neurogenesis and synaptic plasticity in mice. FEBS Lett, 2010. PMID 20600002
• Rescue of cognitive-aging by administration of a neurogenic and/or neurotrophic compound. Neurobiol Aging, 2014. PMID 24702821

This article is for educational and research purposes only and is not medical advice.