TUCSON, Ariz. — After multiple combat tours as a Navy SEAL, Marcus Capone tried talk therapy. Brain-injury clinics. Prescription drugs. Nothing worked to ease his crippling depression and anxiety.

Then he smoked the venom of the Sonoran desert toad.

“I saw why they call this the ‘God molecule’ after I got a full central nervous system reset,” said Mr. Capone, 45, who now runs a nonprofit with his wife helping hundreds of other Special Operations veterans access toad medicine.