Mechanism of action of sedating antihistamines

Addiction, the biggest problem, can usually be avoided by keeping the dose to 0.5 mg BID or less. Fall risk is increased in the elderly by roughly the same proportions.

Other problems include higher incidence of car accidents (risk x1.5), and hip fractures (risk x 1.8) (Ray, Griffin et al. Similarly, low doses of diazepam (Valium) (2 mg) can be quite effective for dizziness.

The term "vestibular suppressant" is a vague one generally used to indicate drugs that reduce nystagmus evoked by a vestibular imbalance or which reduce motion sickness.

Table 1 lists commonly used vestibular suppressants, which consist of three major drug groups, the anticholinergics, the antihistamines, and the benzodiazepines.

There are differential effects across benzodiazepines on Gaba-A receptor subtypes. Addiction, impaired memory, increased risk of falling, and possibly impaired vestibular compensation are their main shortcomings.

Lorazepam and klonazepam are particularly useful agents because of their effectiveness and simple kinetics.

Clonazepam (Klonopin), is as effective a vestibular suppressant as lorazepam (Ganaca et al, 2002).

Table 2 lists the drugs that are commonly used for control of nausea in vertiginous patients.

Unfortunately, astemizole does not appear to be generally useful as it is ineffective in preventing motion sickness (Kohl et al, 1987) and because it has significant potential toxicity.

There is evidence for involvement of several types of histamine receptors.

Phenothiazines, such as prochlorperazine (Compazine) and promethazine (Phenergan), are effective antiemetics, probably because of their dopamine blocking activity, but they also act at other sites. Because these drugs can induce significant side effects, such as dystonia, they are considered second-line drugs whose use should be brief and cautious.

Drugs that speed gastric emptying, such as metoclopramide (Reglan) and powdered ginger root may be helpful in managing emesis (Grontved et al, 1988).

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