Acetaminophen self-administered in the drinking water increases the pain threshold of rats (Rattus norvegicus).

Previous studies have suggested that the addition of flavored acetaminophen suspension (for example, Children's Tylenol) in the drinking water of rats may not be effective in producing postoperative analgesia because of low levels of consumption. However, these investigations neither measured analgesia nor compared the consumption by rats that had undergone surgery with that by unmanipulated rats. The present study reports that although unmanipulated rats naive to the taste of flavored acetaminophen do indeed drink significantly less of this liquid than tap water, they drank sufficient amounts of the acetaminophen-containing solution to significantly raise pain thresholds, as measured by the hot-plate test. Moreover, rats that had undergone surgery drank significantly more acetaminophen solution than did those that had no surgery. These data suggest that oral self-administration of flavored acetaminophen by rats may be an appropriate means to reduce pain.