Negative feedback loops, Tolerance, Dependence & Withdrawal.

I couldn't find the plot that I was looking for, but this electrical plot is equivalent.

From http://www.tpub.com/neets/book9/37k.htm

e_A represents the amount of a substance that perturbs one of the body's negative feedback loops. The amount oscillates between 0V & 100V.

e_R represents the effect of the substance on the body. 100V represents maximum effect and -100V represents maximum anti-effect.

The very first time that the substance is taken, there is 100V of effect, initially. As the time-constant of the negative feedback loop "kicks-in", the effect decays exponentially. Just before the substance is discontinued, the effect is down to 36.8V. Just after the substance is discontinued, the anti-effect is -63.2V. If the input continues to oscillate between 0V & 100V, the effect & anti-effect eventually become equal in magnitude. This is known as "cycling".

If the substance is applied continuously, the effect decays exponentially to 0V. When the substance is discontinued, the anti-effect is -100V initially, but decays exponentially to 0V.

This is analogous to drug tolerance, dependence & withdrawal, where eventually, the user has to take the drug just to feel normal, and discontinuing the drug gives the worst withdrawal symptoms ever, initially. After the drug has been discontinued for a while, the withdrawal symptoms decay exponentially to zero.

The above also applies to supplements that perturb one of the body's Hypothalamic Pituitary NFB loops e.g. the HPA (Adrenal), the HPG (Gonadal) or the HPT (Thyroid) Axes, or any other system (as everything in the body is regulated by a negative feedback loop).

This explains why a supplement can work really well at first, then its effect decays exponentially, until there is zero effect. The loop has compensated for it.

EDIT: If a loop is broken, due to zero secretion of one of the hormones controlling it, then a prescription drug/hormone restores the loop's output level to normal. E.g.

1) Prednisone for a broken HPAA (primary, secondary or tertiary hypoadrenalism) e.g. Addison's Disease.
2) Testosterone (men) or progesterone (women) for a broken HPGA (primary, secondary or tertiary hypogonadism).
3) Levothyroxine for a broken HPTA (primary, secondary or tertiary hypothyroidism) e.g. Hashimoto's thyroiditis.

I'm on 2) & 3), due to a broken pituitary gland. Luckily, it's not completely broken, so I don't need 1).