Beginner's Guide to Peptides: What They Are, How Dosing Works, and What to Check

New to peptides: the three product categories, reading a dose in mcg, mg or IU, reconstitution, storage, injection sites, vendor checks and legal basics.

Written bySebastian Hunter, Founder and EditorI'm a 42-year-old senior advisor and distance runner who came to peptides as a sceptic. I'm not a doctor: I read the research, run my own experiment and keep honest notes. Read full bio

Published September 28, 2026Updated September 30, 2026

The short version

Peptide products fall into three legal categories: FDA-approved medicines, supplements and cosmetics, and unapproved research compounds. Mixing them up is where most confusion starts. This guide shows how to tell them apart, how a dose is written, and what to check before spending money on any of them.

Three categories exist: approved medicines, supplements/cosmetics, and research-only compounds.
Doses are written in mcg, mg or IU, and mixing these up is a common, serious error.
Reconstitution math and syringe units decide the actual dose you inject.
A vendor's research-only labeling is not a safety or legality guarantee.

Peptide products show up in three very different places: a doctor’s prescription pad, a supplement shelf, and a research-chemical vial labeled not for human use. All three get called peptides in casual conversation, and that one word hides a lot. This guide is the ten-minute version of everything else on this site. It covers what category you’re looking at, how a dose is written, and what to check before reconstituting or buying anything.

The three kinds of peptide product

My full compound directory sorts every compound this site covers into one of three buckets. What are peptides? explains the legal reasoning behind them in depth. The short version:

Approved medicines. Drugs like semaglutide and tirzepatide have been through FDA clinical trials and carry an FDA-reviewed label. A licensed pharmacy dispenses them against a prescription. See GLP-1 peptides and FDA-approved peptides: the complete list.

Supplements and cosmetics. Ingested collagen and topical peptide serums are sold without FDA premarket approval. Supplements (ingestion) and cosmetics (topical, appearance claims) each fall under a separate legal framework. See collagen peptides and peptides for skin.

Research compounds. Compounds like BPC-157 and CJC-1295 are sold labeled for research use only, not for human consumption, and are not FDA-approved for any human indication. See research peptides and the law for what that label does and does not mean.

Much of the current interest sits in this third bucket. NPR reported in February 2026 that synthetic peptides are growing popular with biohackers and wellness influencers, taken in the hope of better athletic performance and health. In July 2026 FDA’s compounding advisory committee met to review seven of them, BPC-157 among them, for possible use in compounded medicines. Interest is real and growing. The human trial evidence for most of these compounds is still to come.

How to read a dose: mcg vs mg vs IU

Peptide doses are written in micrograms (mcg), milligrams (mg), or occasionally International Units (IU) for growth-hormone-axis products. Confusing these is the most common and most consequential beginner mistake.

1 mg = 1,000 mcg. A 250 mcg dose and a 0.25 mg dose are the same amount, written two ways.
IU is a different kind of unit, not a fixed weight. For FDA-approved somatropin (Omnitrope), the label states cartridge strength as both figures together: 5 mg/1.5 mL (15 IU) and 10 mg/1.5 mL (30 IU). That works out to 1 mg = 3 IU. The conversion is specific to that product’s label. See HGH 191AA for the detail and why not every product states an IU figure at all.
My glossary defines every unit and abbreviation used across this site in one place.

Reconstitution and syringe units

Most research peptides ship as a freeze-dried powder. You dissolve it into a liquid (reconstitute it) before you can measure and inject it. The arithmetic is exact math, not guesswork: how much diluent, what concentration results, and how many syringe units that equals. Getting it wrong silently changes the dose.

How to reconstitute peptides walks through the equations, worked example included.
Peptide syringe and needle guide explains syringe scales (U-100 vs U-40), why U-50 isn’t a real scale, and needle gauge.
Dosage charts show pre-calculated tables for common compounds and vial sizes.
The dosage calculator does the same math interactively for your own numbers.

Storage

Freeze-dried powder and a reconstituted (dissolved) peptide are two different storage questions. Dissolving a peptide removes the dryness that keeps it stable. Temperature, freeze-thaw cycling, light exposure and the diluent all affect how long a mixed peptide holds up, and no single shelf-life number applies to every peptide. See peptide storage and shelf life for what drives degradation, including what two FDA-approved labels state for comparison.

Injection sites

Subcutaneous injection, under the skin into the fat layer, is the route for most approved injectable peptide medicines and most research compounds alike. The three standard sites are the abdomen, thigh and upper arm. Why rotating between them matters is covered in subcutaneous injection sites, grounded in how FDA-approved injectable drug labels describe their own instructions.

Choosing a vendor

An FDA-approved medicine is dispensed by a licensed pharmacy against a prescription. The pharmacy and the prescription are the quality check. A research compound bought online has no equivalent regulator checking the vial, so vendor evaluation is the buyer’s own job.

Vendors directory: this site’s reviewed vendor listings.
Peptide vendor checklist: what to ask before buying from anyone, including certificate of analysis, testing methods, company transparency and payment protection.
How to read a peptide COA: what a genuine certificate of analysis contains, and how to spot one that isn’t.

Safety and legal basics

Safety and legality both depend on which of the three buckets a product sits in. No single answer covers peptides as one category.

Are peptides safe?: the safety picture by category, including what FDA has documented and what nobody has studied.
Research peptides and the law: what research-use-only labeling means and does not mean, and why an injectable can’t lawfully be sold as a dietary supplement.

Common questions

What is the difference between a peptide medicine, a peptide supplement, and a research peptide?

An approved medicine, like semaglutide, has been through FDA clinical trials and requires a prescription. A supplement or cosmetic, like ingested collagen or a topical peptide serum, is sold without premarket FDA approval under a different legal framework. A research peptide, like BPC-157, is sold labeled for research use only and has not been approved by FDA for any indication. See my full compound directory for how every compound I cover is sorted into these buckets.

Is it safe for a beginner to use peptides?

It depends on which category the product falls into. An FDA-approved medicine has a documented safety profile and is used under a clinician's supervision. A supplement or cosmetic has a long history of ingestion or topical use. A research-only compound has not been through a controlled human safety trial, so its safety in people is largely unknown. See Are peptides safe? for the full breakdown by category.

What should a beginner check before buying a peptide?

For an approved medicine, confirm a licensed pharmacy dispenses it against a real prescription. For a research compound, check for three things. You want a batch-specific certificate of analysis showing both purity and identity testing, a real company address, and no direct human-dosing instructions on the seller's site. See my vendor checklist for the full list.

What is the most common dosing mistake beginners make?

Confusing units. Milligrams, micrograms and International Units are not interchangeable, and a decimal-point or unit error can change a dose tenfold. The second most common mistake is misreading a syringe scale, such as treating a 0.5 mL barrel as if it held 50 units per mL rather than 100. See how to read a dose and the syringe and needle guide below.

References (2 sources)

Bibliographic detail is fetched from PubMed, not written by me - so a citation here cannot drift from the paper it names. Study design comes from PubMed's own tags rather than my judgement.