Handling and preparation
These terms describe the vial in front of you and what you do to it before anything else happens.
| Term | What it means |
|---|---|
| Lyophilised | Freeze-dried. Water removed under vacuum, leaving a stable powder or a thin disc at the bottom of the vial. Sometimes barely visible — a vial that looks empty usually isn't. |
| Reconstitution | Adding liquid to a lyophilised powder to turn it back into an injectable solution. |
| Bacteriostatic water | Sterile water containing 0.9% benzyl alcohol, which prevents bacterial growth and so allows a vial to be entered more than once. |
| Sterile water | Sterile, but with no preservative. Suitable only for single use, because nothing stops bacterial growth after the first entry. |
| Concentration | How much compound sits in each millilitre of solution, written mg/mL. Set entirely by how much liquid you add — the vial's contents don't change. |
| Diluent | The generic term for whatever liquid you reconstitute with. |
Measurement and syringes
This cluster is where most arithmetic mistakes originate, because three different scales are in play at once: mass, volume, and syringe units.
| Term | What it means |
|---|---|
| mg / mcg | Milligram and microgram — units of mass. 1 mg = 1,000 mcg. Mixing these up is a thousand-fold error, and it is the most common one. |
| mL / cc | Millilitre and cubic centimetre — units of volume. They are the same thing; 1 mL = 1 cc. |
| Unit (U) | A mark on an insulin syringe. Not a unit of mass. It is a volume marking whose meaning depends on the syringe. |
| U-100 | A syringe marked 100 units per millilitre. One unit = 0.01 mL. The common type. |
| U-40 | A syringe marked 40 units per millilitre. One unit = 0.025 mL. The same liquid reads as a different number here. |
| Gauge | Needle thickness, counted backwards — a higher gauge number is a thinner needle. Insulin syringes are typically 29–31G. |
| Dead space | Liquid left in the needle hub after the plunger bottoms out. Insulin syringes with a fixed needle have very little; detachable-needle syringes have more. |
Pharmacokinetics
These describe what happens after the injection — how fast the compound arrives, how long it stays, and whether repeated doses build up.
| Term | What it means |
|---|---|
| Half-life (t½) | The time for the amount in your body to fall by half. Independent of how much you started with. |
| Tmax | Time to peak concentration after a dose — how long absorption takes to outpace elimination. |
| Cmax | The peak concentration itself. |
| Steady state | The plateau reached when the amount going in each interval equals the amount cleared. Takes roughly five half-lives. |
| Accumulation ratio | How much higher levels sit at steady state than after a single dose. Above 1 means doses stack; near 1 means each clears before the next. |
| Clearance | The rate at which the body removes the compound. Also takes about five half-lives to be effectively complete. |
| Bioavailability | The fraction of a dose that actually reaches circulation. Near-total for subcutaneous injection, very low for most oral peptides. |
Injection and protocol
The vocabulary of actually running something over time.
| Term | What it means |
|---|---|
| Subcutaneous (subQ, SC) | Into the fat layer beneath the skin. The standard route for peptides. |
| Intramuscular (IM) | Into muscle tissue. Deeper, a longer needle, and a different absorption profile. |
| Site rotation | Deliberately moving where you inject so no one area is used repeatedly. |
| Lipohypertrophy | Thickened, lumpy fatty tissue from repeated injection in one spot. Absorption from an affected area becomes unpredictable. |
| Titration | Changing the amount in planned steps over time rather than starting at the intended level. |
| Cycling | Running a compound for a defined period, then taking a defined break. |
| Stack | Running more than one compound over the same period. |
| Washout | A deliberate gap with nothing administered, long enough for the previous compound to clear. |
Key takeaways
- A syringe "unit" is a volume marking, not an amount of compound — its meaning depends on whether the syringe is U-100 or U-40.
- mg versus mcg is the highest-consequence confusion in the whole vocabulary: a factor of one thousand.
- Concentration is something you choose when you reconstitute, not a property of the vial.
- Five half-lives is the rule of thumb for both reaching a plateau and clearing out.
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Frequently asked questions
Keep reading
This guide is general information for people already organising their own protocol. It is not medical advice, it does not recommend any compound or dose, and Pep AI is not a medical device. Talk to a qualified healthcare professional about anything you inject.
Published by the Pep AI team · Updated August 2026