Tolerance to sedative-hypnotics

Tolerance—decreased responsiveness to a drug following repeated exposure—is a common feature of sedative-hypnotic use. It may result in the need for an increase in the dose required to maintain symptomatic improvement or to promote sleep. It is important to recognize that partial cross-tolerance occurs between the sedative-hypnotics described here and also with ethanol (see Chapter 23)—a feature of some clinical importance, as explained below. The mechanisms responsible for tolerance to sedative-hypnotics are not well understood. An increase in the rate of drug metabolism (metabolic tolerance) may be partly responsible in the case of chronic administration of barbiturates, but changes in responsiveness of the central nervous system (pharmacodynamic tolerance) are of greater importance for most sedative-hypnotics. In the case of benzodiazepines, the development of tolerance in animals has been associated with down-regulation of brain benzodiazepine receptors. Tolerance has been reported to occur with the extended use of zolpidem. Minimal tolerance was observed with the use of zaleplon over a 5-week period and eszopiclone over a 6-month period. 

There is an important difference between tolerance to narcotics and tolerance to sedative-hypnotics. As noted earlier, toler-... 

Development of tolerance to sedative hypnotics is common and leads to escalating doses... 

The development of tolerance is a major drawback to the use of benzodiazepines in the long-term treatment of insomnia. Whereas tolerance to the hypnotic effects of benzodiazepines permits them to be used without excessive sedation when treating anxiety disorders, this is counterproductive when attempting to treat insomnia. Patients often find themselves requiring higher doses to obtain the same sedative-hypnotic effect initially accomplished by lower doses. For this reason, careful consideration must be given before benzodiazepines are used to treat chronic insomnia. 

Chronic insomnia calls for careful assessment for a medical cause, non-pharmacologic treatment, and careful use of sedative-hypnotics (intermittently to prevent tolerance and dependence). 

Like the barbiturates, the benzodiazepines make it easier to fall asleep and to stay asleep through the night. However, they also suppress REM sleep, which can lead to REM rebound when they are discontinued. Tolerance to their sleep-promoting effects often develops after chronic use. Some long-acting benzodiazepines, such as flurazepam (Dalmane), are associated with pronounced hangover effects in the morning and are therefore problematic as sedative-hypnotics. Others, with a short-to-intermediate dnration of action, are more desirable as hypnotics. 

Patients with ethanol or sedative-hypnotic overdose may be euphoric and rowdy ("drunk") or in a state of stupor or coma ("dead drunk"). Comatose patients often have depressed respiratory drive. Depression of protective airway reflexes may result in aspiration of gastric contents. Hypothermia may be present because of environmental exposure and depressed shivering. Ethanol blood levels greater than 300 mg/dL usually cause deep coma, but regular users are often tolerant to the effects of ethanol and may be ambulatory despite even higher levels. Patients with GHB overdose are often deeply comatose for 3-4 hours and then awaken fully in a matter of minutes. 

Longer-term difficulties associated with benzodiazepine use for insomnia come from observations that many patients develop tolerance for these agents, so that they stop working after a week or two. To avoid this, patients must take a sleeping pill only a few times within several days, or for only about 10 days in a row followed by several days or weeks with no drug treatment. Furthermore, if patients persist in taking benzodiazepines as sedative-hypnotics for several weeks to months, there can be a withdrawal syndrome once the medications are stopped, particularly if they are stopped suddenly. This is discussed in further detail in Chapter 13. 

Drug combinations Probably the most common combination treatment is concomitant use of an SSRI and a benzodiazepine, especially on initiation of treatment (Fig. 9—6). The benzodiazepines (especially alprazolam and clonazepam) not only appear to act synergistically to increase the onset of therapeutic action and perhaps even boost the efficacy of SSRIs, but they also appear to block the anxiogenic actions of the SSRIs and lead to better tolerability as well as the ability to attain therapeutic dosing levels for the SSRIs. Sometimes sedative-hypnotics such as zale-plon or zolpidem are required in addition to an SSRI, especially on initiation of SSRI treatment. 

Barbiturates (a class of drugs with more effective sedative-hypnotic effects) replaced bromides in 1903. Depending on the dose, frequency, and duration of use, however, tolerance, physical dependence, and psychological dependence on barbiturates can occur relatively rapidly. With the development of tolerance, the margin of safety between the effective dose and the lethal dose becomes very narrow. That is, in order to obtain the same level of intoxication, the tolerant abuser may raise his or her dose to a level that can produce coma and death. 

Regular use of the sedative-hypnotics may result in tolerance—the need for increasing doses to achieve the same effect. Within two to four weeks, tolerance can develop to the sedative effect of minor tranquilizers taken at night for sleep. Thus, these drugs are not usually used prescribed for more than a few days at a time. 

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