Re: How do antiemetics work?

I have restated your questions and tried to answer them individually.

How do antiemetics work? (Reference 1)
Several classes of antiemetic drugs are now available.  These agents block 
the actions of different neurotransmitters (histamine, acetylcholine, 
serotonin and dopamine) in the body.  These agents act at several sites in 
the complex processes involved in the vomiting reflex.  The vomiting reflex 
is coordinated in the vomiting center in what is called the "lateral 
reticluar formation", which is found in the medulla oblongata (a part of 
the central nervous system present at the base of your brain).  This region 
receives information from various parts of the body, including the 
"chemoreceptor trigger zone" (which is present in the brain), the 
vestibular system (part of which is formed by the parts of the middle ear) 
and from various sites in the stomach and intestines.


Do dramamine and meclizine act in different ways on the body? (reference 1)
Dimenhydrinate (DRAMAMINE) and meclizine (ANTIVERT) both block certain 
actions of histamine , as well as certain actions of acetylcholine, in the 
body. 


How do they prevent nausea, vomiting and dizziness? (reference 1)
Antihistamines and anticholinergics (agents that block the action of 
acetylcholine) may be effective in the treatment of motion sickness as the 
result of a similar action: the ability to block the transmission of 
information from the vestibular apparatus (the part of the middle ear that 
is involved in balance) to the emetic center in the medulla oblongata, 
which is a part of the brain involved in coordinating various reflexes 
(e.g., swallowing, vomiting).


Why is drowsiness a usual side effect? (reference 1)
Histamine, acting via H1 receptors in the central nervous system, increases 
wakefulness.  Therefore, antihistamines that block the binding of histamine 
to H1 receptors and which enter the central nervous system cause 
drowsiness.  In addition, anticholinergic agents cause drowsiness and, 
thus, antihistamines which possess anticholinergic activity also produce 
drowsiness via this action.  Thus, older agents, such as diphenhydramine, 
which enter the central nervous system, cause sedation, while newer 
antihistamines, such as astemizole (HISMANAL) and loratadine (CLARITIN), 
which poorly penetrate into the CNS, are nonsedating.  Patients vary in 
their susceptibility to the sedative effects of antihistamines.


Do all antihistamines have the same effect on motion sickness? (reference 
1)
Not all "antihistamines" are effective in the treatment of motion sickness. 
 The antihistamines that are effective for the treatment of motion sickness 
all have significant anticholinergic activity and, thus, it may be the 
anticholinergic (rather than antihistaminergic) action of these drugs that 
make them effective for treating motion sickness.  The most effective of 
the antihistamines for the treatment of motion sickness are: dimenhydrinate 
(DRAMAMINE), diphenhydramine (BENADRYL), promethazine (PHENERGAN) cyclizine 
(MEREZINE) and meclizine (ANTIVERT).

It should be pointed out that, although the term "antihistamine" is 
commonly thought to refer to medications that are useful for treating 
allergies and motion sickness, it actually refers to any drug that blocks 
the action of histamine.  Histamine, like many substances in the body, 
exerts a variety of effects, depending on the tissue in which it is acting 
and the type of "receptor" with which it interacts.  There are three types 
of histamine receptors: so called, H1, H2 and H3 receptors.  The agents 
that are effective for treating motion sickness are H1 receptor antagonists 
(with significant anticholinergic actions).


I have heard ginger (which acts on the stomach rather than the nervous 
system) is also effective... what's the most effective? 
There are varying reports on the effectiveness of ginger as an anti-nausea 
and antiemetic agent.  Gary P. Barnas, MD, Medical Director of the Medical 
College of Wisconsin International Travelers Clinic states, "Recent studies 
have shown that Ginger root may be as effective as the other drug 
treatments [for motion sickness] but is associated with fewer side 
effects." (reference 2).  In contrast, a study by Stewart and colleagues 
concludes, "…ginger does not possess antimotion sickness activity" 
(reference 3).

I am not certain which is the most effective medication for the treatment 
of motion sickness.  It seems that there is significant variability in 
patient responses to the available drugs and in the severity of the side 
effects that these drugs produce.

The U.S. Food and Drug Administration states, "The Food and Drug 
Administration considers four active ingredients to be safe and effective 
for use in OTC drugs for motion sickness, says Gerald Rachanow, deputy 
director of the monograph review staff in FDA's Office of OTC Drug 
Evaluation. The ingredients are cyclizine
(Marezine and others), dimenhydrinate (Dramamine and others), 
diphenhydramine (Benadryl and others), and meclizine (Bonine and others). 
(reference 5)




Finally, I suggest that you take a look at the following MadSci page for a 
description of motion sickness:
 http://www.madsci.org/posts/archives/may99/926654723.Ns.r.html




REFERENCES
1 Goodman & Gilman's The Pharmacological Basis of Therapeutics, Ninth 
Edition (1996), edited by J.G. Hardman et al., pp. 928-932


2 http://healthlink.mcw.edu/article/907367055.html


3 Stewart J.J., Wood, M.J., Wood, C.D. and Mims, M.E.  (1991)  Effects of 
ginger on motion sickness susceptibility and gastric function.  
Pharmacology 42 (2), pg. 111-120.


4 http://www.hc-sc.gc.ca/hpb/lcdc/publicat/ccdr/96vol22/dr2213ea.html

5 http://www.fda.gov/opacom/catalog/tummy.html