Medically how much substance left, is considered inconsequential?

Generally in medicine, after a substance, like a drug, with a certain half-life, is it considered an inconsequential amount anymore? Something that will no longer affect your body. Is it like 10%, 5%, 1%, less, more? I know there's a general rule of thumb in statistics that anything below 5% is inconsequential, but that's just a rule of thumb, is there a more proper number within the medical community that they accept?