Desensitisation is the concept that, with repeated expose, a receptor will become less active when the same concentration of agonist is present. This is frequently found to occur in individuals with a dependence on a drug.
With individuals experiencing drug addictions, this results in the need for higher and higher doses over time. This can become unsafe, especially with drugs that have a narrow therapeutic window - as the dose gets higher, it becomes more likely that it will become fatal. Due to the diminished response by the body to increasing doses, the patient may not realise that the dose is becoming harmful.
One example is with individuals administering opiates. These are widely used analgesic drugs (especially in the US and North American), however doses must be raised over time as the analgesic effect reduces. This is due to desensitisation. This has caused particular problems in countries with medical marketing issues, including the United States, as many patients are now dependent on opioids. The widespread use of these drugs has also allowed unregulated use, with some doctors and patients providing these compounds to individuals without the need for the strong pain relieving properties. This has caused an ‘epidemic’ of opioid addictions, and is yet to be resolved.