Today I learned that human body metabolizes different drugs not only at different elimination speed, but also with with different acceleration of the elimination.
1) Zero-order kinetics is linear.
For example, alcohol is eliminated from human body and a constant volume rate, about 1 drink per hour.
That means that the higher alcohol concentration is - the longer alcohol elimination half-life is (due to saturation of metabolism of alcohol).
At high alcohol concentration, liver is not able to quickly eliminate alcohol from human body.
https://www.perplexity.ai/search/zero-order-kinetics-DtaruZCYSRejV7hDJq74hg#0
Zero-order kinetics describes reactions or elimination processes where the rate is constant and independent of reactant or substrate concentration.
2) First-order kinetics is exponential.
For example, Ozempic (semaglutide) half-life in human body is about 7 days, no matter whether Ozempic concentration is low or high.
3) Second-order kinetics is approximately proportional to the square of the drug concentration.
That means that:
- if drug concentration is higher, then the drug half-life is shorter.
- if drug concentration is lower, then the drug half-life is longer.
For example, vitamin B-12 half-life is about 7 days at normal concentrations, but half-life can go up to 20-30 days at lower concentration of B-12.
Another example is levothyroxine (T4):
- At normal T4 concentration - metabolism half-life is 7 days.
- At high T4 concentration - half-life is 2-3 days.
- At low T4 concentration - half-life is 9-12 days.
In case of vitamin B-12 and thyroxine T4, human body quickly eliminate excess, but is very slow at metabolizing already low concentration of these hormones.
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