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For instance half life of a 150ml drug injected to you is 50 days (so you have 75ml left in your system). At 100 days you'll have 37.5ml left in your system. But does that mean the drug is most effective as well on the first 50 days? And only becomes half as effective between 50 days to 100 days?

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    Of course, the drug effectiveness will drop along with its concentration in your body, but the association is not necessary linear. It depends on a drug. For example, you can increase the effectiveness of an antibiotic by increasing the dose, but after a certain dose, effectiveness will not continue to increase. – Jan Jan 7 at 11:40
  • The relationship doesn’t need to be linear: If you double the fuel of a plane, the plane won‘t have exactly twice as much range, but a bit less because it needs a bit of the added fuel to support the weight of the additional fuel. – Narusan Jan 7 at 16:31
  • @Jan That seems a bit mechanistic and oversimplified. Eg: Does your depiction still fit the commonly agreed MoA for lysergic acid & friends, in fact many psychoactive-drugs, like prozac, hypericin? – LаngLаngС Jan 11 at 0:37
  • I wanted to say that it depends on a drug how its effectiveness will change with the dose and time, so I couldn't see how the question could be answered. After re-reading, I see that the question focuses on the half-time... – Jan Jan 11 at 8:35
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Two key serum concentrations to know about in pharmacodynamics and pharmacokinetics are the Minimum Effective Concentration (MEC) and Toxicity Concentration (TC).

Some medications have stronger effects the higher the concentration, and other medications have the same effects regardless of the concentration as long as it is above the MEC. Note that the ceiling is either the Toxicity Concentration, or the concentration at which adverse effects often arise.

That brings us to the concept of "therapeutic window" which are concentrations between MEC and TC.

The EFFECT of the medication entirely depends on which medication you're discussing. But generally, the interval and dosing of most medications are designed specifically to keep it inside the therapeutic window.

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