What does liquid methadone do to the body?

Same as pills: The liquid is just the vehicle for the drug, although many have food colorings and sugar which may cause problems in some people. Long-term methadone has few major side effects when used properly, but causes physical dependency (like all opioids), and can reduce both male and female sex hormones, cause constipation, dry mouth/dental issues, sweating, weight gain, and rarely a heart rhythm issue.