where λ z is the elimination rate constant, presented in Eq. (3.10) (also see Figure 3.6).Because t 1/2 and λ z are inversely proportional, a compound with a long half-life will have a long, flat terminal phase, corresponding to a small λ z or terminal slope value, similar to the profile show in Figure 3.6 b. Based on the 68 hour half-life of azithromycin, here is an approximate concentration chart of azithromycin in your body relative to time after the last dose: 1 Half-life: 68 hours (~3 days): 50% 2 Half-lives: 136 hours (~6 days): 25% The half-life is the amount of time it takes for a given isotope to lose half of its radioactivity. If a radioisotope has a half-life of 14 days, half of its atoms will have decayed within 14 days. In 14 more days, half of that remaining half will decay, and so on. Half lives range from millionths of a second for highly radioactive fission Absorption half-life 1 h, elimination half-life 12 h. intranasal oxytocin has a central duration of at least 2.25 hours and as long as 4 hours. The half-life of a drug is an estimate of the time it takes for the concentration or amount in the body of that drug to be reduced by exactly one-half (50%). The symbol for half-life is t½. For example, if 100mg of a drug with a half-life of 60 minutes is taken, the following is estimated: Hydromorphone: 2-3 hours; Methadone: 15-20 hours; Methadone’s Half-Life. You may notice that methadone has an elimination half-life significantly greater than any of the other opioid medications listed above. In fact, some sources have indicated that methadone’s half-life is as long as 36 hours. The half-life of meth can vary depending on several factors including the route of administration, but methamphetamine’s half-life has been found to be about: 1. 25 hours in the urine. 10 hours in the blood. If you took 200 mg of a drug with a half life of 1 hour: 100mg remains after 1 hours. 50mg remains after 2 hours. 25mg remains after 3 hours. 12.5mg remains after 4 hours. 6.25mg remains after 5 hours. Almost 97% of the drug is eliminated after 5 hours. This is usually enough to consider the drug's effects negligible. The half-life equal to the dosing interval at steady-state where the maximum concentration at steady-state is twice the maximum concentration found for the first dose and where the fall off to the trough concentration from the maximum concentration is consistent with this half-life. 2. Terminal half-life: t 1/2,z =0.693/λ z: The half-life gWnX.

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