...To add: you may be confusing over-the-counter pain relievers with narcotic painkillers like hydrocodone (Vicodin), oxycodone (Oxycontin/Percocet), morphine, & codeine. Through an unknown mechanism, these drugs have shown to cause an increase in appetite in some instances (although in instances of opiate/opioid dependency, high tolerance, & abuse--the alternative is usually present: loss of appetite). But I repeat, typical pain relievers (or NSAIDs) do not
act on mu-opioid receptors like narcotics do, nor do they act on dopamine & norepinephrine like stimulant-appetite suppressants do; so pain relievers do NOT cause similar effects or antagonist effects. Drugs like aspirin & ibuprofen act as inhibitors of COX enzymes, & this causes minor pain relief! This is their only major neurological effect.
For some reference:
Since hydroxycut is a dietary supplement, its ingredients are hard to discover! Originally its main ingredient was ephedrine, which is a substituted amphetamine AND a methamphetamine analogue (although it does NOT have similar abuse potential or ability to cause a "high" or euphoria). But, the FDA outlawed ephedrine in the U.S. a few years ago (due to its action as a vital precursor in manufacturing meth). So, ever since, hydroxycut does NOT contain ephedrine.
Instead, hydroxycut uses a combination of substances, caffeine and green tea extract being the main substances responsible for hydrocut's effects. There are other ingredients included in the formulation that do one of three things: 1) aide in the stimulatory-appetite suppressing action (of caffeine/tea extract); 2) affect metabolism and/or metabolic rate through a different, non-stimulatory mechanism; or 3) attempt to aide overall health by including vitamins & nutrients. Such substances known to be included in hydroxycut are: garcinia cambogia (extract), glucomannan, L-carnitine, willow bark (extract), alpha lipoic acid, chromium, potassium and calcium.