How this calculator works
Methodology and sourcing
Still In My System estimates how much of a substance is mathematically left in your body using exponential decay, the same model used to describe radioactive decay and most single-compartment drug elimination. The formula is:
You give the calculator three things: which substance, how much you took, and exactly when you took it. We look up that substance's published half-life, work out how much time has passed since your dose, and run that through the formula above. The decay chart and timeline you see afterward are just that same formula evaluated at many points along the way, plotted against the real clock times rather than abstract durations, because "9:35 PM" is something you can actually picture and "1.8 hours" generally isn't.
Where the half-life numbers come from
Each substance page links to its source in the "what is the half-life" section, and the same citation lives in our underlying data file. We prioritize FDA prescribing information and peer-reviewed pharmacokinetic studies over secondary blog summaries. Where the literature reports a range rather than a single number — caffeine's half-life, for instance, is commonly cited anywhere from 2 to 12 hours depending on the person — we use the most frequently cited midpoint and say so on that substance's page, rather than picking whichever number makes for a cleaner chart.
What this calculator can't know
Real elimination is not actually a single clean exponential curve for every person. It depends on liver and kidney function, age, body composition, genetics (caffeine metabolism alone varies roughly six-fold across the population depending on CYP1A2 activity), other medications you're taking, and — for orally absorbed substances — how full your stomach was. None of that is visible to a calculator that only knows a dose and a timestamp. We built this to answer "roughly where am I on the decay curve," not "exactly how many milligrams are in my bloodstream right now."
Who maintains this
This site is maintained as a small, focused reference: five substances, kept current, rather than a sprawling database stretched thin. If you spot a half-life figure that looks out of date or a calculation that seems off, the contact page reaches a real person who can check it.
This is a mathematical estimate, not medical advice. See our terms of use for the full disclaimer.