TENS also known as transcutaneous electrical nerve stimulation is used to reduce pain by stimulating the opioid mechanism in the same way as pain relief such as morphine but without the difficulty of getting your pet to take pills- we've all been there!
This leads to an increase in endogenous opioids in the spinal cord which reduces the transmission of pain signals. For your pet to feel pain the signal must be passed through the spinal cord, when TENs or opioid pain killers are used they block the transmission of these pain signals along the spinal cord meaning that your pet no longer feels pain.
Due to this, TENS has a longer carry over period than other forms of pain relief and is therefore often used in chronic cases. This means that treatments are needed less often and the effects last longer than oral medications.