If you want to flush your system I would start with a good recommended cleansing of your bowel. Then cleanse the kidneys and liver with taking milk thistle capsules. This is in the case to rid your body of years of unwanted toxins of metals, etc. Make sure to drink plenty of water and eat light meals, not fatty or high meats..Fruits is very good for the first day and lots of water and rest.
Why do you want to do this, will effect what method, but keepin mind- doing the detox in that order of colon, kidney and liver that will keep you from feeling ill effects of cleansing. Also you may get a slight headache the first day-feel a little foggy in the head. This will suffice after a day or two- if you feel worse as the days carry on- stop whatever you are taking.
If you have diabetes don't do this though without doctors okaying it as well if you are managing any condition. For the case of heavy metals ridding- I would recommend a different way of cleanse or you could get pretty sick.
However if this is because of drug use. Stop using the drugs.