Peptides Explained

The same handful of questions come up every time someone finds out I research this stuff: what actually is a peptide, is it the same as a steroid, how is it taken, is it safe, and is it even legal here. This page answers all of that in plain English, once, properly. It's the beginner's version. If you are coming at this from the gym rather than the science, start with what peptides actually are, written for Irish gym-goers — the same ground in five minutes, including what gets sold online versus what the research supports. When you want the detail on a specific compound, that's what the full research library further down the page is for.

What exactly are peptides?

A peptide is a short chain of amino acids, the same building blocks your body uses to make every protein you're built from. String enough amino acids together and you get a protein; a shorter chain (roughly 50 amino acids or fewer) is called a peptide. That's the whole distinction. It's length, not some exotic separate class of chemical.

Your body already makes thousands of peptides on its own, and it uses them as signals. Different peptides carry instructions for different jobs, including:

• Immune function — telling immune cells when to switch on.
• Muscle and tissue repair — helping the body rebuild after damage.
• Metabolism — regulating appetite, blood sugar and how energy gets used.
• Growth — signalling to the growth hormone system.
• Structure — collagen, for example, is what holds skin, tendon and joint tissue together.

Therapeutic or "research" peptides are lab-made versions designed to copy or support one of those natural signals. That's why they sit much closer to hormones than to steroids, and it's worth being clear on that early, because the two get lumped together constantly and they don't work the same way.

How do peptides work?

They work as signals, not as fuel. A peptide binds to a receptor on a cell, the receptor passes a message inside, and the cell does something differently — releases a hormone, starts repairing, changes how it handles energy. That is the whole mechanism, and it is why peptides sit closer to hormones than to steroids.

Two things follow from that. First, a peptide can only do something your body already has the machinery to do; it turns up a signal that already exists rather than adding a new one. Second, the receptor decides the effect, not the compound’s reputation. Ipamorelin and GHRP-6 both act at the same growth hormone receptor, but GHRP-6 drives appetite hard and Ipamorelin barely touches it. Same family, different result.

What are the main types of peptide?

Peptides aren't one thing with one effect. They're usually grouped by the outcome researchers are interested in. Here's the plain-English map, with an example compound or two for each. Where there's a full guide in the library below, the name links to it.

One honest caveat: the strength of the evidence is very different across that table. Semaglutide and tirzepatide have large human trials behind them. A lot of the recovery, longevity and cognitive compounds are backed mostly by animal studies and early-stage work. Don't assume a fact about one peptide applies to another just because they're both "peptides". Read the individual guide before you draw conclusions.

Which peptides are studied for skin?

GHK-Cu is the one with genuine independent research behind it. It is a copper-binding tripeptide the body already makes, studied for collagen production, wound healing and gene expression, and it appears in cosmetic formulations as Copper Tripeptide-1. AHK-Cu is its close relative with far thinner evidence — most of what is known comes from a single hair-follicle study.

Collagen peptides are the odd one out: taken orally, absorbed well, and the only peptide in this section most people will ever buy in a shop.

One distinction worth holding on to: most of the GHK-Cu literature is topical or in cell culture. Applying something to skin and injecting it are not the same question.

Which peptides are studied for muscle growth?

Honestly, none of them have convincing human evidence for building muscle in healthy trained people.

The growth-hormone-axis compounds — Ipamorelin, Sermorelin, CJC-1295, the GHRPs, MK-677 — do reliably raise growth hormone and IGF-1. That part is measured and not in dispute. Whether raising those numbers in an already-healthy adult produces meaningful extra muscle is a different question, and the answer is much less flattering. MK-677 has a two-year trial in older adults that raised IGF-1 and maintained lean mass, and reported no improvement in functional measures.

Follistatin 344 is sold on myostatin inhibition, but the human data comes from gene therapy trials rather than the injectable protein. MGF and PEG-MGF rest on splice-variant biology observed in rodents.

The SARMs get asked about in the same breath and belong in a separate box: they act on the androgen receptor, not the GH axis, and MK-677 is filed with them everywhere despite not being one. RAD-140 is the most marketed of them and has no randomised performance trial at all — its only human study was a phase 1 in cancer patients, and its main finding was liver enzyme elevation. See the full SARMs page and the HardCore Supplements review for where this is actually sold in Ireland and what the disclosure says.

The boring answer is still the true one. A calorie surplus, enough protein and progressive overload do more than any of this, and they cost nothing.

How are peptides made?

Worth understanding, because it partly explains why sourcing quality matters so much. Peptides used in research and medicine come from three broad routes:

The method itself isn't really the risk. A peptide can be made perfectly and still be a gamble if it's then handled by an unregulated supplier with no real quality control. Purity is the thing that actually matters, and I'll come back to that under safety.

How are peptides taken?

Most peptides are given by subcutaneous injection — a small injection into the fat layer just under the skin. The reason is simple: peptides are proteins, and the digestive system breaks proteins down, so swallowing most of them doesn't work. Oral bioavailability (how much actually makes it into your bloodstream) is often only around 1–2%. Collagen peptides are the notable exception, absorbing at roughly 63% orally, which is why collagen is one of the few you'll genuinely see sold as a powder or capsule.

Injectable peptides usually arrive as a freeze-dried powder that has to be reconstituted (mixed into liquid) before use. The general handling principles researchers follow:

• Reconstitute with bacteriostatic water — sterile water with a small amount of preservative that keeps it usable for longer.
• Don't shake it. Add the water gently down the side of the vial and let it dissolve; shaking can damage the peptide.
• Store the mixed vial in the fridge at around 4°C.
• Once reconstituted, it's generally used within about a month.

The full step-by-step version — bacteriostatic water versus sterile water, equipment, storage, and the mistakes beginners make — is in the peptide reconstitution guide.

This site does not publish dose, volume, or syringe-draw calculations for any compound — that's a conversation for a pharmacist or doctor, not a fitness website.

Are peptides safe?

The honest version, not the sales-page version. A few things are worth understanding before anyone goes near this:

• Immunogenicity. Because peptides are biological signals, the immune system can sometimes react to them — injection-site reactions and immune sensitivity are among the more commonly reported issues. Individual response varies a lot.
• The growth hormone question. Peptides that push the growth hormone system (GH secretagogues) are generally considered inappropriate for anyone with a history of cancer, because of the theoretical concern around encouraging cell growth. This is a genuine contraindication, not a formality.
• Most reported effects are mild and short-lived for the peptides that have been studied — but "studied" is doing a lot of work in that sentence, and for many compounds the human safety data simply isn't there yet.
• Purity is the real-world risk. With research-grade peptides bought online, contamination and inaccurate dosing from unregulated suppliers are far more common problems than anything inherent to the peptide itself.

None of the above is a green light. It's context. If you're seriously considering anything here, the sensible order is bloodwork and a conversation with a doctor first, purchase later — not the other way around. I had that order wrong for years myself; why I test before I change anything now is the short version of why I stopped guessing. Speak to a qualified healthcare professional who knows your history before making any decision.

What are the side effects of peptides?

It depends entirely on which peptide, and for a lot of them nobody knows, because the human safety data does not exist. Where side effects are documented, these are the ones that come up repeatedly:

• Injection site reactions — redness, soreness, swelling. The most commonly reported issue across almost every injectable here.
• Nausea and appetite loss — dominant in the GLP-1 class, and the main reason people stop taking them.
• Fluid retention, joint pain and carpal tunnel symptoms — documented with growth hormone itself, and the reason the GH-axis compounds are watched for the same pattern.
• Appetite spikes — GHRP-6 in particular, strongly enough that it is the defining feature of the compound.
• Headaches and flushing — mild and short-lived in the studies that report them.

Melanotan II deserves naming on its own. It has no approval from any medicines regulator anywhere, and the documented concerns include new or changing moles and pigmented lesions, priapism, and case reports of acute kidney injury from unregulated injectable use.

And the risk nobody puts on a list: what is actually in the vial. With unregulated suppliers, contamination and inaccurate content are more likely to cause harm than the peptide’s own pharmacology.

Are peptides legal in Ireland?

This is more complicated than a yes/no, and I'm not a solicitor, so treat this as context rather than legal advice. The picture splits roughly in two:

• GLP-1 medicines are prescription-only. Semaglutide (Ozempic, Wegovy) and tirzepatide (Mounjaro) are approved, regulated medicines. In Ireland they fall under the Health Products Regulatory Authority (HPRA) and can only be obtained legally through a prescription from a doctor. Buying them any other way isn't a grey area — they're prescription medicines.
• Most other research peptides are unapproved. The large majority of the compounds people are curious about (BPC-157, TB-500, the GH-axis peptides, and so on) have no licensed consumer pathway in Ireland. They're typically sold as "research chemicals, not for human consumption", which puts them in a legal grey area rather than a clearly regulated one.

A word of caution that follows directly from that: because those research-grade compounds aren't regulated as medicines, purity and quality control can be genuinely poor. There's no regulator checking that what's on the label is what's in the vial. That's the single biggest practical risk with unapproved peptides, and it's worth weighing heavily.

(You'll sometimes see American sources describe peptide regulation in terms of the FDA and "503A/503B" compounding pharmacies. That's US law and it doesn't apply in Ireland — here the relevant body is the HPRA.)

What is coming next?

Peptides are one of the faster-moving areas in health research right now. The GLP-1 category alone has gone from a diabetes treatment to one of the most significant weight-loss developments in decades, and newer multi-receptor compounds are still working through clinical trials. Beyond metabolism, there's active research into peptides for recovery, immune support and age-related decline — though a lot of that is still early, and early research has a habit of looking more certain in a headline than it does in the data.

The honest summary: it's a genuinely promising field, the tools for delivery and design keep improving, and it's also one where the hype runs well ahead of the evidence for most compounds. The useful move as a beginner isn't to chase the newest thing — it's to understand the fundamentals (like the ones on this page), get your basics right first, and read the actual research on anything specific before acting.