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Decay rate for radionuclides are well known and predictable according to the standard equation for radioactive decay that can be found in any physics textbook. The decay of a radionuclide is completely unaffected by physical variables (temperature, pressure, physical state, etc.). Pharmacological half lives are far more complex because a drug substance undergoes more than a simple metabolic process. In the case of Sinemet, the drug first has to be ingested, then be converted into a permeable substance and absorbed into the circulatory system for transport to the brain and then has to get through. The kinetics of these reaction are not simple as the rate determining step has to be that final one. Bio equivalence is most likely measured within the vascular system which may or may not be identical to the concentration of dopamine in the brain sites. I tend to get lost in these games and no matter the rationale for nourishing the hungry synapses, I am at wit's end to help my Barbara to cope with on/off periods and wild dyskenesias. I rely to a very large extent on our knowledgeable medical expert, and try to keep our chins up. Love Michel