Wearing out the receptors in Laymans terms is like taking to much caffiene or Ephedra and wearing out your addrenal glands.
If you start out by taking 1 fat burner a day, within 2 weeks you will probably need 2 pills to get the same energy rush .
If a person takes to many drugs or doesnt give his body a break the body becomes saturated with that drug.Therefore it no longer repsnds to the drug, or it takes twice the dosage to get the same results.
Another good example is I knew a guy who took pain medication.He would take 3-4 pills a day, but after a year he had to take 8-10 of them to get the same effective.
The body has adapted to the feeling and now has built a tolence to the medicatio he was using, but then abusing.
Steroids like ay drug,if over used the body builds a tolerance to.Once this happens you have to stop using them for atleast 6 months, or increase your dosage