Often times, the very condition the drug is used for, is eventually a catalyst that increases the effects of the disorder it treats.
For example, opiate ultimately cause opiate induced hyper algesia, opiate used
for too long actually cause pain. Benzodiazapines relieve anxiety, taken long enough, they cause restlessness and anxiety. Sleep aids eventually cause insomnia. This is due to the neuro receptors and chemical production of the brain adjusting to the foreign chemical.