Some more dangerous fat burners are Wildfit Quest ones that are pretty much an energy drink in a pill. Not to say all energy drinks are bad, but the ones that have a lot of caffeine in them with carbonation are not too healthy. Diet pills that have similar ingredients are equally harmful. You may start to feel things like the jitters and possibly shaking.
Sleep apnea and broken sleep are also effects. Consequences like this happen if you take these substances and decide not to do anything afterwards. They are stimulants and make your body feel like it has energy. The problems come when it wares off or if you don't burn it out of your system.At this stage, you will feel very sluggish and down.
Over time of continuous use, problems might become long term and depending on how your body operates, life threatening. Another dangerous pill that has flooded the market and become illegal in some countries is phentermine. This is pretty much a hard drug and should not be confused with other more safe fat burning supplements like Phen375.