Peptide FAQ: Straight Answers to Common Questions
Thirty straight answers to the peptide questions people actually ask: dosing units, mixing, storage, timelines, legality, safety, vendors and goals.
The short version
This page collects the questions people type into a search box about peptides. They cover how doses are measured, whether mixing compounds is fine, how long a vial lasts, whether any of this is legal, and which peptide goes with which goal. Each gets a few plain sentences and a link to the full page with the sourcing.
- Unit confusion (mcg vs mg vs IU) and syringe-scale errors are the most common real mistakes.
- Legality and safety both depend on which of three product categories you are asking about.
- No page here recommends a vendor. It explains how to evaluate any seller yourself.
People search for straight answers. This page collects the questions I see most often, grouped by topic. Each gets a few plain sentences and a link to the page with the full sourcing. Nothing here is medical advice. See are peptides safe? and research peptides and the law for the fuller legal and safety picture behind the short answers.
Dosing and units
What is the difference between mcg, mg and IU?
Milligrams (mg) and micrograms (mcg) are units of mass: 1 mg equals 1,000 mcg. International Units (IU) measure biological activity, not a fixed weight. The label for FDA-approved somatropin (Omnitrope) states cartridge strength in both, which works out to roughly 1 mg = 3 IU. That conversion is specific to that product. See the beginner’s guide and glossary for every unit used on this site.
How do I convert a peptide dose into syringe units?
Divide the target dose by the vial’s concentration in mcg/mL to get a volume in mL. Then multiply by 100 for a U-100 syringe, since 100 units equals 1 mL. How to reconstitute peptides walks through the full equations with a worked example, and the dosage calculator does the same math interactively.
What does U-100 mean on a syringe?
U-100 means the syringe is graduated so that 100 units equals 1 mL. It describes the scale, not the barrel size. Barrel sizes of 0.3, 0.5 and 1.0 mL are separate capacity choices on the same U-100 scale. See the peptide syringe and needle guide for the full breakdown, including U-40 syringes.
Is there such a thing as a U-50 syringe?
No. What gets called a U-50 syringe is usually a 0.5 mL barrel that holds 50 units. It is still graduated at 100 units per mL, so it is a true U-100 syringe, just smaller. Treating it as 50 units per mL halves the intended dose, a documented dosing error covered in the syringe and needle guide.
Does adding more water to a vial create more peptide?
No. The total mass of peptide in the vial is fixed. Adding more diluent only lowers the concentration and increases the volume needed per dose. The number of doses a vial yields is total peptide mass divided by dose size, not how much water was added. See how to reconstitute peptides.
Mixing and reconstitution
What is bacteriostatic water, and do I need it?
Bacteriostatic Water for Injection, USP, is sterile water containing benzyl alcohol as a preservative. That is why a multi-dose vial can be entered more than once under label conditions. Ordinary sterile water for injection contains no preservative and is generally framed for single-use contexts. See bacteriostatic water for label specifics including concentration and pH.
Can I mix two peptides together in the same vial or syringe?
No controlled human trial has tested specific peptide combinations yet. Combining compounds adds unknowns around interaction, purity and dosing error rather than reducing them. See peptide stacking for why people stack, what the combination research covers, and how common-practice stack protocols are labelled.
What needle should I use to draw up and inject a peptide?
A drawing needle used to pull solution from a vial is sometimes a different, larger-gauge needle than the one used to inject. Withdrawing liquid is a different job from a comfortable small injection. Fine, short needles in roughly the 29-31 gauge range are typical for subcutaneous injection. The exact gauge and length for any specific product is set by that product’s own packaging. See the syringe and needle guide.
How do I dispose of used needles and syringes?
FDA describes a two-step process. First, place the used needle or sharp into a sharps disposal container immediately after use, keep it out of reach of children and pets, and never reuse it. Second, once the container is about three-quarters full, dispose of it following your community’s guidelines, such as drop-off at a participating pharmacy. Full detail is on the syringe and needle guide.
Storage
How long does a reconstituted, mixed peptide last?
No single shelf-life number applies to every peptide once it is dissolved in liquid. It depends on temperature, light exposure, the diluent used, concentration, and the specific peptide’s own chemical stability. See peptide storage and shelf life for the variables that decide this, including what two FDA-approved labels state for comparison.
Should I refrigerate or freeze a mixed peptide?
Refrigeration generally slows the chemical reactions that degrade a peptide. Freezing is not automatically better. Freeze-thaw cycling can physically damage a peptide through aggregation, which is why some approved peptide-drug labels tell patients not to freeze the product even though it is otherwise stored cold. See peptide storage and shelf life for the specifics.
Does light exposure matter for peptide storage?
Yes. Photodegradation is a recognized route by which some peptide bonds and side chains break down. That is the general reason pharmaceutical storage instructions for injectable peptide products commonly include protecting the product from light as well as heat. See peptide storage and shelf life.
How long peptides take to work
How long does it take for a peptide to start working?
There is no single answer; it depends on the category. Approved prescription peptides follow whatever their clinical trials and label describe. Products like collagen are studied over multi-week to multi-month trial windows. For most research peptides no trial-measured human timeline exists yet, so the timelines you see come from user reports and common practice. See how long do peptides take to work?
How long should a peptide cycle be?
It depends on the specific compound and use. Any cycle length quoted for a research peptide is common practice reported by sources, not a trial-established protocol. Check the compound’s own dosing page, reachable from the compounds directory, for the sourcing behind any cycle-length figure.
Legality and safety
Is it legal to buy research peptides?
It depends what you mean by legal. Selling a compound labeled for research use only is a different question from whether a consumer can lawfully use it as a drug. Federal law also says an injectable cannot lawfully be marketed as a dietary supplement. See research peptides and the law for the full legal reasoning.
What does research-use-only labeling mean legally?
It is the seller’s stated basis for selling the product as a laboratory chemical rather than a drug. It is not a government safety rating or proof of purity. FDA has stated in warning letters that it looks at a product’s whole marketing picture, not just the label. It uses that picture to decide whether the product is intended as a drug for human use. See research peptides and the law.
Are peptides banned in sports?
Some are and some are not, depending on the compound. Research peptides such as BPC-157 and growth-hormone secretagogues including CJC-1295 and ipamorelin are prohibited at all times under the current WADA Prohibited List. An ingested collagen supplement is not a banned substance. Check the compound’s own page, such as BPC-157, for its listed WADA status.
Can a doctor prescribe me a research peptide like BPC-157?
No, not lawfully, including through telehealth. A prescription requires a legal drug. Research peptides such as BPC-157 are not FDA-approved and are generally not on the list of bulk substances that may lawfully go into a compounded drug either. See telehealth peptide prescribing for what a clinician can and cannot lawfully prescribe.
Are peptides safe?
It depends entirely on the category. Collagen and topical cosmetic peptides are, on current evidence, low-risk. FDA-approved prescription peptides have real documented effects and side effects, which is why a clinician is involved. For most research-only compounds, long-term human safety data does not exist yet. See are peptides safe? for the full breakdown.
What are common peptide side effects?
It depends entirely on the compound. Side effects for an FDA-approved GLP-1 medicine are documented in its label. For most research peptides no systematic human safety data exists yet, so there is no trial-based side-effect list to draw from. Check the compound’s own page, reachable from the compounds directory, for what is and is not documented for it.
Where should I inject a peptide?
FDA-approved injectable drug labels describe three standard subcutaneous sites. These are the abdomen (avoiding roughly a two-inch ring around the navel), the front or outer thigh, and the back or outer upper arm. Rotate between doses to avoid tissue changes at one spot. See subcutaneous injection sites for the full anatomy and technique picture.
Why does rotating injection sites matter?
Repeatedly injecting into the exact same small spot is linked to tissue changes there over time. These are lipohypertrophy, a thickened lump of fat tissue, or, less commonly, lipoatrophy, a dent from lost fat tissue. That is why approved injectable drug labels tell you to rotate between sites with each dose. See subcutaneous injection sites.
Vendors and quality
How do I find a legitimate peptide vendor?
Check for a batch-specific certificate of analysis showing both purity (by HPLC) and identity (by mass spectrometry). Check for a real named company with a checkable physical address. Check that there are no direct-to-consumer dosing instructions, which are themselves a legal red flag. See the peptide vendor checklist for the full list. This site does not sell peptides or endorse any specific vendor.
What is a certificate of analysis (COA)?
A COA is a document a laboratory issues after testing a specific batch or lot of material. It reports what was tested, which method was used, and what the lab found. A genuine one names the peptide identity, batch number, purity method, storage conditions the results apply to, and the issuing lab. See how to read a peptide COA for what a genuine one contains and how to spot one that is not.
Does a low price mean a vendor is untrustworthy?
Not by itself. It is one of the pricing checks worth applying alongside the certificate-of-analysis and transparency questions in the vendor checklist, and should not be treated as a bargain on its own.
Does PeptideDecode sell or recommend a specific vendor?
No. This site does not sell peptides and does not endorse or rank any specific vendor. The vendors directory lists vendors reviewed against the criteria in the vendor checklist. Neither page is an invitation to buy from anyone.
Choosing a peptide for a goal
Which peptide is used for weight loss?
The FDA-approved medicines are semaglutide and tirzepatide, both GLP-1-class prescription drugs with their own clinical trial data and labels. See GLP-1 peptides for the approved options, and the evidence-ranked weight-loss peptides for everything else tagged to this goal, evidence level included.
Which peptide is used for muscle or recovery?
This spans prescription and research-only products with very different evidence bases. See the muscle goal hub and the recovery goal hub for everything tagged to each. Every card carries its own evidence level and bucket, so a rodent finding and a human trial are never shown as equivalent.
Which peptide is used for skin?
Oral collagen peptides have the most human evidence of any category on this site. Topical peptides are a mixed bag. The single best-evidenced ingredient is palmitoyl pentapeptide-4 (Matrixyl), while copper peptides (GHK-Cu) have weaker direct trial support. See collagen peptides and peptides for skin for the detail.
Which peptide is used for anti-aging or longevity?
See the aging goal hub for every compound and guide tagged to this goal on the site, each shown with its own evidence level. Much of the anti-aging peptide research is still preclinical or unconfirmed in humans, and the hub says so rather than implying a product works because it is listed.