Tolerance with 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. 

The CCK system shares one property with the opioid system, ie, the existence of selective nonpeptide antagonists. These include aspedicine, a natural benzodiazepine (136), and Devazepide (L-364,718 MK-329) (137). Selective, potent peptide antagonists for CCK, eg, Cl-988 and PD 134308, have been developed that maybe useful as anxiolytics and as dmgs which increase the analgesic effect of morphine but at the same time prevent morphine tolerance (138) (see Hypnotics, sedatives, anticonvulsants, and anxiolytics). 

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. 

Unlike benzodiazepines, buspirone is not associated with sedative or abuse problems, but some clinicians have observed that bus-pirone s anxiolytic properties do not appear to be as potent as those of benzodiazepines, particularly in patients who have previously received a benzodiazepine. Because buspirone is not sedating and has no psychomotor effects, it has a distinct advantage over benzodiazepines when optimal alertness and motor performance are necessary. Response to buspirone occurs in approximately 2-4 weeks. Buspirone does not show cross-tolerance with benzodiazepines and other sedative or hypnotic drugs such as alcohol, barbiturates, and chloral hydrate. Therefore, buspirone does not suppress benzodiazepine withdrawal symptoms. In anxious patients who are taking a benzodiazepine and who require a switch to buspirone, the benzodiazepine must be tapered gradually to avoid withdrawal symptoms, despite the fact that the patient is receiving buspirone. 

Nontolerant individuals who consume alcohol in large quantities develop typical effects of acute sedative-hypnotic drug overdose along with the cardiovascular effects previously described (vasodilation, tachycardia) and gastrointestinal irritation. Since tolerance is not absolute, even chronic alcoholics may become severely intoxicated if sufficient alcohol is consumed. 

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. 

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. 

Nonbarbiturate sedative-hypnotics have a similar mechanism of action as barbiturates and have high potential for tolerance, abuse, dependence, overdose, and withdrawal reactions. Chloral hydrate is still commonly used today due to its efficacy as a short-term sedative hypnotic and low cost. Chloral hydrate should not be used in patients with severe renal, hepatic, or cardiac disease. 

Sedative-hypnotics are prescribed for short-term use because the patient can develop a tolerance of and dependency on the medication. Avoid chronic use of sedative-hypnotics. Nonpharmacological methods that promote sleep should be tried before prescribing sedative-hypnotics to aid with sleep. 

Despite the advantages of benzodiazepines compared with other sedative-hypnotic drugs, tolerance and dependency do occur. Rebound insomnia may also occur either during each night of treatment with benzodiazepines or during withdrawal of treatment extended for several days. Cessation of benzodiazepines may also lead to recurrence of the original symptoms or even to transient worsening. 

Sedative -Hypnotics - Midazolam (8) continues to receive substantial support as an effective hypnotic with neither tolerance effects nor rebound insomnia on drug withdrawal. A series of papers has addressed safety and psychomotor performance aspects of this drug. The clinical literature concerning the hypnotic effi-... 

Older medications, such as the barbiturates, are used as sedative-hypnotics, but toxicity limits their widespread use. For example, they can cause significant central nervous system (CNS) depression, physical dependence, and tolerance. Additionally, they are potent inducers of liver enzymes, which can lead to clinically significant drug interactions when these medications are administered with other drugs extensively metabolized by the liver. 

Benzodiazepines produce little respiratory depression. Dosages of >2000 mg produce lethargy, drowsiness, confusion, and ataxia. However, the effect of CNS depression is additive when taken with ethanol, barbiturates, or other sedative-hypnotics. Also, the euphoric effects are lower than those experienced with most other sedative-hypnotics. Tolerance, risk of dependence, or addiction is relatively low. Tolerance can still... 

Carbromal (Uradal, Adalin.) 1 mole of a-bromo-a-ethyl butyryl bromide is mixed with dry urea (1 mole) and heated on a steam bath for several hours. Precautions must be taken to keep steam and atmospheric H2O from the reaction vessel. Cool, allow to solidify, wash with H2O, and recrystallize from alcohol. Dose (sedative) 300 to 500 mg, (hypnotic) 700 to 950 mg, mp 116-118°. This drug is less potent than the barbiturates, but it is less toxic, extremely well tolerated, has a wide margin of safety, and acts rapidly. 

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