TENS units still need Food and Drug Administration (FDA) approval for treating constipation. This doesn’t mean you can’t use them for this purpose.
TENS units are Class II medical devices, which require manufacturers to prove to the FDA that their devices are as safe and effective as other similar devices. They don't need to prove that the devices are effective for treating a specific condition.
Several manufacturers have received clearance to sell TENS devices. Before you buy, check to see whether the FDA has cleared the device you’re interested in for sale.