Acetaminophen
How acetaminophen produces
analgesia is not entirely known. A proposed mechanism of action of acetaminophen
is that it inhibits a form of the cyclooxygenase enzyme known as COX-3. (See section
on NSAIDs for a more detailed description of the COX enzyme system) COX-3 is only
found within the central nervous system, which might help explain why acetaminophen
has analgesic and antipyretic (fever reducing) properties, but does not have the
anti-inflammatory effect of NSAIDs. Despite having very little anti-inflammatory
effect, acetaminophen may be an effective analgesic for some inflammatory conditions.
Acetaminophen has fewer side effects than do NSAIDs. It has no effect on platelet
function, rarely causes gastrointestinal problems, and can be given to individuals
who cannot take aspirin or other NSAIDs. However, acetaminophen can produce severe
hepatotoxcity or liver damage in some patients, even at recommended therapeutic
doses. Patients at greatest risk of hepatotoxicity when taking acetaminophen include:
- People who abuse alcohol
or who consume large amounts of alcohol (more than 3 alcoholic beverages a
day). Since 1998, the FDA has required alcohol warnings for acetaminophen
and other nonprescription (over-the-counter) analgesics.
- Individuals who are malnourished
or fasting
- Patients with preexisting
liver disease
- Individuals who are also
using other drugs that can cause liver damage
To prevent hepatotoxicity,
the total dose of acetaminophen should not exceed 4 gms or 4000 mg in a 24-hour
period.
There are multiple trade
names of acetaminophen-containing medications, including both prescription and
over-the-counter drugs. As a result, patients may accidentally take higher-than-recommended
doses of acetaminophen. Therefore, patients should be taught to look for acetaminophen
in prescription and over-the-counter analgesics, and other types of medications
such as cold remedies.
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Acetaminophen
has fewer side effects than NSAIDs.