Absorption and half-life
In Yamamoto and colleagues' pharmacokinetic study, peak plasma concentrations occurred within about 2.5 hours and the estimated plasma half-life was about 35 hours. These are observations from a particular study, rather than an individual timetable.
Half-life means the time required for a concentration to fall by half during elimination. After one half-life, about half remains; after two, about one quarter remains under a simplified model. Complete disappearance does not occur at the end of the first interval.
Accumulation with repeated exposure
Repeated administration can add drug before the previous amount has been eliminated. This overlap produces accumulation. A person can therefore have a different exposure after several days than after the first administration.
With consistent intervals, concentrations may approach a repeating pattern called steady state. They still fluctuate within that pattern. Changing a schedule or stopping administration does not immediately remove the amount already present.
Sleep and next-day effects
Difficulty sleeping may persist beyond the period when someone intended to feel stimulated. Taking a long-acting substance earlier cannot guarantee that its effects will be absent at bedtime.
Coffee, energy drinks, decongestants and pre-workout products can add their own effects. Sedating products may make a person feel sleepier without correcting an abnormal rhythm or an electrolyte disturbance. Persistent insomnia is a reason to review the exposure, rather than add another substance to balance it.
Persistent symptoms
Symptoms depend on more than the amount of drug remaining. Low potassium, dehydration or a cardiac complication can require treatment in their own right. An average half-life cannot determine whether an irregular heartbeat is safe to wait out.
The Spiller poison-center series described prolonged toxicity, including effects lasting beyond a day. Chest pain, fainting or severe breathlessness require urgent assessment regardless of the time since use.
Blood and urine testing
Blood and urine tests examine different specimens and use methods with different detection limits. A laboratory may detect drug after obvious stimulation has faded. The reverse inference is also unreliable: symptoms do not reveal a measured concentration.
Repeated use, sample timing and the analytical method affect detection. No universal negative-test date follows from the plasma half-life. Competitive athletes should use their anti-doping organization's process rather than an estimated off period.