Plain-English definitions of the terms behind every compound profile and calculator on Halflife Labs — from half-life and steady state to reconstitution and titration. Each term links to the relevant tool or compound data.
- Half-life (t½)
- The time it takes for the active concentration of a compound in the bloodstream to fall by 50%. It governs dosing frequency and how long a compound stays in your system. Model any compound's decay with the half-life calculator or browse values in the compound database.
- Steady state
- The equilibrium reached when the amount of a compound entering the body equals the amount being cleared, so concentrations plateau. It typically takes about 4–5 half-lives of consistent dosing. See how to calculate steady state.
- Cmax (peak concentration)
- The maximum plasma concentration a compound reaches after a dose. Higher Cmax can mean stronger peak effects and, for some compounds, more pronounced side effects shortly after injection.
- Tmax (time to peak)
- The time after administration at which Cmax (peak concentration) occurs. Tmax depends on the route of administration and the compound's absorption rate.
- Trough
- The lowest concentration of a compound in the dosing interval, reached just before the next dose. The days-5–7 trough on weekly GLP-1s is why some users feel effects fade before their next injection.
- AUC (area under the curve)
- A measure of total compound exposure over time, calculated as the area under the concentration-versus-time curve. AUC reflects how much active compound the body is exposed to across the whole interval.
- Elimination / clearance
- The processes by which the body removes a compound, mainly via the kidneys (renal), liver (hepatic), and enzymatic breakdown. Clearance rate, together with dose, determines steady-state concentration.
- First-order kinetics
- Elimination in which a constant fraction (not a fixed amount) of the compound is cleared per unit time. Most peptides and GLP-1 medications follow first-order kinetics, which is what makes half-life a meaningful, constant value.
- Bioavailability
- The fraction of an administered dose that reaches systemic circulation in active form. Intravenous dosing is 100% by definition; subcutaneous, intramuscular, oral, and intranasal routes are typically lower.
- Protein (albumin) binding
- The reversible attachment of a compound to plasma proteins such as albumin. High albumin binding (as engineered into tirzepatide and several long-acting insulins) dramatically extends half-life by shielding the compound from clearance.
- Reconstitution
- Dissolving a lyophilised (freeze-dried) peptide with a diluent — usually bacteriostatic water — to create an injectable solution at a known concentration. Get exact volumes with the reconstitution calculator.
- Bacteriostatic water (BAC)
- Sterile water containing 0.9% benzyl alcohol, which inhibits bacterial growth and allows a reconstituted vial to be used over multiple days. It is the standard diluent for reconstituting peptides.
- Subcutaneous (SC) injection
- An injection into the fatty layer just under the skin, the most common route for peptides and GLP-1 medications. Rotating sites prevents tissue damage — track it with the injection site rotation tracker.
- Titration / dose escalation
- Gradually increasing a dose over weeks to improve tolerability, standard for GLP-1 medications. Build a personalised schedule with the GLP-1 dose escalation calculator.
- Ester
- A chemical group attached to a hormone (such as testosterone) to slow its release and extend half-life. Longer esters like cypionate and enanthate allow less frequent injections. See testosterone cypionate.
- GLP-1 receptor agonist
- A class of compounds that activate the GLP-1 receptor to reduce appetite and improve glycaemic control, including semaglutide and tirzepatide (a dual GIP/GLP-1 agonist).
- GH secretagogue
- A compound that stimulates the body's own release of growth hormone, rather than supplying GH directly. Includes GHRH analogues like CJC-1295 and ghrelin-mimetics like ipamorelin.
- Lipohypertrophy
- Lumps of thickened tissue caused by repeatedly injecting the same site, which impairs absorption. Rotating injection sites prevents it — see the rotation tracker.