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Mercury (as methylmercury) in the body has a half-life of about 65 days. Lead in the blood has a half life of 28–36 days. [29] [30] Lead in bone has a biological half-life of about ten years. Cadmium in bone has a biological half-life of about 30 years. Plutonium in bone has a biological half-life of about 100 years.
An effective half-life of the drug will involve a decay constant that represents the sum of the biological and physical decay constants, as in the formula: = + With the decay constant it is possible to calculate the effective half-life using the formula:
In pharmacology, clearance is a pharmacokinetic parameter representing the efficiency of drug elimination. This is the rate of elimination of a substance divided by its concentration. [ 1 ] The parameter also indicates the theoretical volume of plasma from which a substance would be completely removed per unit time.
For example, the medical sciences refer to the biological half-life of drugs and other chemicals in the human body. The converse of half-life (in exponential growth) is doubling time. The original term, half-life period, dating to Ernest Rutherford's discovery of the principle in 1907, was shortened to half-life in the early 1950s. [1]
The remaining drug and its metabolites are excreted in the faeces via biliary elimination. Less than 1% of the drug is excreted in its unchanged form. [133] After an oral dose of trazodone, it was found to be excreted 20% in the urine as TPA and conjugates, 9% as the dihydrodiol metabolite, and less than 1% as unconjugated mCPP.
Most of the drug is metabolized; very little diazepam is excreted unchanged. [29] The elimination half-life of diazepam and also the active metabolite desmethyldiazepam increases significantly in the elderly, which may result in prolonged action, as well as accumulation of the drug during repeated administration. [114]
The elimination half-life of cetirizine ranges from 6.5 to 10 hours in healthy adults, with a mean across studies of approximately 8.3 hours. [9] [8] The elimination half-life of cetirizine is increased in the elderly (to 12 hours), in hepatic impairment (to 14 hours), and in renal impairment (to 20 hours). [8]
Clotiazepam is metabolised to hydroxy-clotiazepam and desmethyl-clotiazepam. After oral ingestion of a single 5 mg dose of clotiazepam by three healthy volunteers the drug was rapidly absorbed. [15] The elimination half-life of the drug and its metabolites range from 6.5 hours to 18 hours. Clotiazepam is 99 percent bound to plasma protein. [15]