What is the longest drug to stay in your system?

Several psychedelic phenethylamines have very long durations close to 36 hours, like DOB.

Some opiates like Methadone and Buphrenophine can last 36-48 hours.

Other psychedelic drugs can last for over 24 hours and LSD can last up to 12.

It's hard to say which lasts the "longest" though, just because there's so many different groups of psychoactive drugs. A better question would be "whats the longest lasting opiate" or "whats the longest lasting psychedelic drug" ect.

Hope this helped.