Cyclic glycine-proline (cGP) is a tiny ring-shaped peptide that the body makes when it breaks down IGF-1, the growth hormone-driven growth factor.[1][11] It is also found in New Zealand blackcurrants.[1][3] Human studies link higher cGP levels to better stroke recovery and memory, but they are observational, and no placebo-controlled trial has tested cGP itself.[5][6] It is not an FDA-approved drug.
What is cyclic glycine-proline?
cGP is a cyclic dipeptide: two amino acids, glycine and proline, joined into a ring. It is likely formed from glycine-proline-glutamate (GPE), a three-amino-acid piece clipped off the start of IGF-1.[11] It is absorbed when taken by mouth: in rats it worked when given orally and even passed to pups through breast milk.[10]
The same molecule is known in Russian research as cyclo-prolylglycine (CPG). Scientists at the Institute of Pharmacology in Moscow identified it in rat brain in 1996 and found it had memory-enhancing effects in rats.[12][18] Much of the cGP research, though, comes from Jian Guan’s group at the University of Auckland in New Zealand.[1] cGP also occurs in marine microbes and other natural sources.[18]
How cGP works: the IGF-1 link
Most IGF-1 in the blood is bound to a carrier protein, IGFBP-3, which keeps it inactive. cGP comes from the part of IGF-1 that binds IGFBP-3, so it competes for that binding spot and can free up IGF-1.[1]
In cell and animal studies, cGP boosted IGF-1 activity when IGF-1 was low and dampened it when IGF-1 was too high.[2] For that reason, the New Zealand group uses the cGP/IGF-1 ratio in blood as a rough marker of how much IGF-1 is active.[1]
Russian studies point to other effects too: cycloprolylglycine had antidepressant-like effects and raised BDNF gene activity in the brains of mice bred to show depression-like behavior.[14]
What the research shows in people
All human cGP research so far is observational or small and open-label:
| Study | People | Finding |
|---|---|---|
| Stroke (New Zealand) | 34 patients, 50 controls | Lower cGP after stroke; higher cGP/IGF-1 ratio at admission linked to better 90-day scores[5] |
| Parkinson’s and cognition | 178 patients, 23 controls | Healthy older adults with a higher ratio had better cognitive scores[6] |
| Pregnancy follow-up | Women 6 years after a first pregnancy | Women with high blood pressure had lower cGP[7] |
| Alzheimer’s brain tissue | 15 Alzheimer’s and 15 control brains | More bound cGP and less free IGF-1 in Alzheimer’s brains[8] |
The Parkinson’s cognition study found that a rising cGP/IGF-1 ratio with age went with normal thinking, while a falling ratio went with dementia, but it measured each person only once.[6] Associations like these cannot show that cGP causes better outcomes.
Blackcurrant supplementation trials
In an open-label New Zealand study, men with Parkinson’s disease took blackcurrant anthocyanin capsules for 28 days, and 11 of them gave spinal fluid and blood before and after. The capsules contained cGP, and cGP rose in their spinal fluid, while spinal fluid IGF-1 did not change.[3] Spinal fluid cGP tracked blood cGP, suggesting that cGP from the diet can reach the brain.[3]
Participants also reported better emotional well-being on a quality-of-life questionnaire.[4] The author noted that the extra attention during the study, or a placebo effect from a “healthy” fruit supplement, could explain that change.[4] A review by the same group also reported lower anxiety scores but no improvement in movement.[1]
Animal research
- Memory: cGP improved spatial memory and reduced amyloid plaques in Alzheimer’s model mice, in a study by a team in India, outside the New Zealand group.[11] Rat pups whose mothers got cGP had better recognition memory.[10]
- Blood pressure: in obese rats on a high-fat diet, cGP lowered systolic blood pressure and abdominal fat without changing overall body composition.[9]
- Brain injury and tumors: cGP improved recovery after brain injury in rats and slowed lymphoma growth in mice.[2]
- Mood: Russian studies found antidepressant-like effects in mice.[14]
cGP and Noopept
Noopept (GVS-111), a nootropic developed at the Zakusov Institute and used in Russian medical practice, is broken down in the rat brain into cyclo-prolylglycine.[13][15] The Russian researchers argued that this metabolite, which is identical to the body’s own cGP, accounts for part of Noopept’s memory effects.[13] This has not been shown in people.
cGP vs IGF-1 LR3
People sometimes group cGP with IGF-1 products, but they are very different. IGF-1 LR3 is an engineered, long-acting form of IGF-1 itself, injected to boost growth signaling, and IGF-1 analogues are banned in sport.[19] cGP is a tiny natural metabolite that is thought to rebalance how much IGF-1 is active, and in animals it raised IGF-1 activity only when it was low.[2]
Dosage: what was studied
There is no established dose of cGP. The main human study used blackcurrant anthocyanin capsules daily for 28 days.[3] In rats, mothers received 3 mg/kg by mouth each day.[10]
A synthetic cGP analog, NNZ-2591, is being developed as a drug. In a phase 2 study, 18 children with Phelan-McDermid syndrome took it by mouth twice a day for 13 weeks, at doses raised from 4 to 12 mg/kg; it was generally well tolerated, but the study had no placebo group.[16] NNZ-2591 is a different molecule, so its results do not apply directly to cGP supplements.
Side effects and safety
- Natural compound: cGP is made by the body and found in foods, but that does not prove high-dose supplements are safe.[1]
- No formal safety trials: side effects of cGP supplements have not been studied in controlled trials.
- IGF-1 balance: too little IGF-1 is linked to poor healing, while too much can help tumors grow.[2] cGP is thought to push IGF-1 activity toward normal in both directions, but this has not been confirmed in people.
- Pregnancy and children: no human safety data exist.
Is cyclic glycine-proline legal?
- New Zealand: sold in blackcurrant-based supplements (cGPMAX) by The cGP Lab, an Auckland company, according to a 2022 trade report.[1][17]
- United States: cGP is not an FDA-approved drug. Supplement rules vary, and products are not checked by FDA for effectiveness.
- Sports: cGP is not named on WADA’s 2026 Prohibited List. Athletes should check any supplement with their anti-doping organization.[19]
This is general information, not legal advice. For other brain-focused compounds, see our cognitive peptides page.
What users report
We did not find user reports on cGP from forums or review sites that meet our standards. Most experience claims online come from sellers, which we do not use.
