The color change is a trick
The pads contain wood or bamboo vinegar. Overnight it reacts with your foot sweat and turns dark. That is the entire trick.
NPR sent used pads to a lab in 2008. The lab found no heavy metals. The brown is a chemical reaction, not extracted toxins. (WebMD)
Leave a pad in a steamy bathroom with no foot attached. It still darkens.
The evidence base is zero
No published clinical studies show these pads remove toxins from the body. None show they are safe, either. The evidence base is not weak. It is empty.
Regulators already settled this
The FTC charged manufacturers millions of dollars for false health claims. The FDA issued warnings on imported brands.
This is not an open scientific question. It is a closed regulatory one. (Wikipedia)
What actually detoxes you
Your liver and kidneys. They filter waste around the clock, for free, with no pads required.
Any product that claims to help them is selling you a job your organs already do.
This guide is general information, not medical advice. If symptoms stick around or worry you, talk to a doctor.
Questions, answered
Why do detox foot pads turn black?
Sweat reacts with the wood vinegar in the pad. It is chemistry, not toxins leaving your body.
Are detox foot pads dangerous?
No published safety data exists, which is itself a problem. The FDA has warned about some imported brands.
Do I need to detox at all?
No. Your liver and kidneys handle waste continuously. You do not need a product to do their job.