A single dose of Clonidine has an elimination half-life of about 12-16 hours. 40-60% of the drug is excreted in the urine unchanged. In people with reduced kidney function, the drug takes longer to clear the kidneys - the elimination half-life is 41 hours.
Since it takes about four days of continuous use for most drugs to achieve a steady state plasma level, it is likely that it will take just as long or longer to eliminate all traces of them from the system. The drug concentration declines slowly over several days. Clonidine is a very potent drug. It should be discontinued with great care under a physician's close supervision. Withdrawal symptoms can be seen as early as 12 hours after the last dose and persist for 5-7 days.