The Sport Pilot rule permit pilots of Light-Sport-Aircraft to forgo a formal FAA medical examination and use your state driver’s license to establish medical fitness to fly an airplane light-sport aircraft (LSA). You must also follow any restrictions or limitations listed on your driver’s license. In addition, you must determine before each flight that you are medically fit to operate your airplane LSA in a safe manner. As a pilot, it is your responsibility to ensure that your current medical health in no way jeopardizes the safety of a flight.
If you are a non-current pilot who has let their medical expire, you can fly as a sport pilot using your drivers license as medical eligibility. In other words, if you have a private pilot licence/certificate, and you let your medical expire, you can use your drivers license as medical eligibility to fly as a sport pilot (only in LSA).
However, as a previous pilot or student pilot, who has had their FAA medical certificate or most recent medical application denied, revoked, withdrawn, or suspended by the FAA, you are not allowed to operate using your driver’s license until you clear the denial from your airman record by having a valid third class medical certificate issued.
Words of wisdom: It is best to let your medical expire and fly as a sport pilot in LSA rather than denied, revoked, withdrawn, or suspended and have to clear medical or not fly at all.