Most second-generation antihistamines do not cause drowsiness, although some (such as cetirizine and fexofenadine), may be more likely to do so at higher dosages.
My doctor also mentioned this to me but at the time I shrugged it off.
The American Society of Anesthesiologists defines the continuum of sedation as follows: In the United Kingdom, deep sedation is considered to be a part of the spectrum of general anesthesia, as opposed to conscious sedation.
Prior to any oral sedation methods being used on a patient, screening must be done to identify possible health concerns.
Second generation antihistamines were developed in the 1980s and are much less sedating than first-generation antihistamines.
They act on histamine-1 receptors in the periphery and are unlikely to penetrate the brain, so are less likely to cause side effects or interact with drugs.
They also act on muscarinic, alpha-adrenergic, and serotonin receptors.