TENS send electrical pulses to the muscles. High frequency (90-120 Hz) relax the muscles, low frequency (2-10 Hz) make them contract. I use such a device and it really helps with back and other muscular pains. it is a natural painkiller that acts on the cause and not the symptom. AFAIK, they are perfectly safe if used as instructed.
Shortest joke: "We may not be able to cure MS but we can manage its symptoms."