Within the bodybuilding community the concept of half-lives is an often misunderstood one. A large part of this confusion can be attributed to the typically complex explanations that accompany this topic. Consequently, I’m going to stay on task and keep this definition/explanation of half-lives very simple. I won’t segue onto drug comparisons and contrasts, nor will I tangent onto the topic of esterification or even delve in great detail into the body’s process of drug metabolism. There are plenty of writings about those related subjects should you choose to study them, but the topic of this piece is quite simply ‘understanding half-lives’.

Essentially, and often contrary to the traditional explanation, a medication’s half-life can be an easy reference for the total time a drug remains active (to varying degrees) within the body. More specifically, this information is used as a guide for optimizing your dosing schedule in order to avoid unwanted peaks and valleys in medication effects, and the often undesirable side effects which accompany such uneven up and down levels.

All medications possess an active life, or total time during which they perform their curative activities within the body. Each drug, even within the same class, has a different (regardless of how similar) chemical structure, which largely determines its half-life. From the moment a drug enters the bloodstream it becomes subject to the three metabolic functions: 1) absorption - work and operation within the body; 2) degradation - breakdown of its chemical structure in preparation for evacuation; and 3) elimination - evacuation/removal from the body.

A drug’s half-life is the time in which it takes for the three metabolic processes to impact a given drug to the extent that one half (50%) of said drug is no longer present within the body’s system. Again, since drugs have varying chemical structures this timeframe will also vary. Sometimes there are stark similarities within classes of drugs, e.g. oral steroids typically possess what we consider to be short half-lives of often between 4-16 hours. However, this is merely a generalization over most of the class, as some other types of orals fall outside these parameters for different reasons.

This half-life is reduces the drug content by 50% every time it is experienced, i.e. after the first half-life you're at 50%, the second takes you to 25% or half of the remaining fifty, the third 12.5%, and so on until the last half-life known as the terminal one occurs...generally accepted as the 8th (below .05%). Although metabolites still exist, thereby making detection possible, the drug's actions or active life has effectively ceased. However, as long as you continue taking a drug prior to the realization of the terminal half-life, you reinitiate its property actions thereby resetting the half-life quotient to a full 100%. Also of importance here is the fact that the more medication you take the greater the concentration in your blood becomes, until you achieve full blood serum saturation. Some people believe achieving full saturation somehow alters the half-life principle, but it doesn’t. Saturation has to do with optimizing the effects you receive from a medication over time, but it still succumbs to the principle of half-life because even though you can have more of drug in your system (amount-wise), you can never exceed the maximum of 100%. Consequently, whenever you reach the hundred percent mark be it the first drug administration or after taking it for several weeks, months or years once the first half-life expires the drug amount is necessarily reduced to 50%.

Summary

That in a nutshell, so to speak, is what a half-life is all about. The amount of time it takes to for a drug to be reduced to one of half of its actual dosage, in a process that happens repeatedly until the drug is no longer effective.

References

Garg U, Jacobs DS, Grady HJ, et al. Therapeutic drug monitoring. In: Jacobs DS, Oxley DK, Demott WR, eds. Jacobs & Demott Laboratory Test Handbook. 5th ed. Cleveland, Ohio: Lexi-Comp, Inc; 2001:731-771.

Mimaki T. Clinical pharmacology and therapeutic drug monitoring of zonisamide. Ther Drug Monitor. 1998;20:593-597.

Moyer TP, Shaw LM. Therapeutic drugs and their management. In: Burtis C, Ashwood E, Bruns D, eds. Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. 4th ed. Philadelphia, PA: Saunders; 2005:1237-1280.

Pippenger CE. Principles of therapeutic drug monitoring. In: Wong SHY, ed. Therapeutic Drug Monitoring and Toxicology by Liquid Chromatography. Boca Raton, FL: CRC Press; 1985:11-36.