Toxicology Dose Back-Calculator

Real oral bioavailability is rarely complete, but assuming 100% (bioavailability = 1.0 in Dose = Concentration × Vd × Body Weight ÷ Bioavailability) produces the smallest possible dose estimate consistent with the measured blood level. That makes the reported dose a conservative floor — the actual ingested amount was likely higher if absorption was less than complete — which fits this tool's role as a worst-case forensic screening estimate rather than a precise reconstruction.

The calculator doesn't model absorption and distribution phases mathematically; it applies a flat 1.3x multiplier to the measured concentration as a rough placeholder for the higher peak that likely occurred before elimination and distribution reduced it. It's explicitly a rough approximation, not a pharmacokinetically modeled peak — real peak reconstruction needs time-since-ingestion data and the drug's specific absorption profile.

It rearranges the first-order elimination equation C(t) = C0 × e^(−ke×t) to solve for t at a target concentration of 0.01 mg/L, giving t = −ln(0.01/C0) / ke. Because it only uses the current measured concentration and the elimination rate you enter, its accuracy depends entirely on how closely your input ke matches the actual drug's kinetics and the individual's metabolism.

Vd (L/kg) reflects how a drug partitions between blood and other body compartments — ethanol's Vd of about 0.7 tracks total body water because it stays largely in blood and water, while fentanyl's Vd of about 4.0 reflects heavy concentration in fat and tissue rather than circulating blood. A higher Vd means a given blood concentration corresponds to a much larger total-body dose, since most of the drug sits outside the bloodstream at any moment.