A dose of local anaesthetic, jut like any other medication, will wear off (be metabolised) after a certain amount of time. However, if more is added through the cannula, the anaesthetic effect will last longer - adding more is topping up. For example, say a dose of medication lasts for 4 hours. If you're given another dose towards the end of the 4 hours, you will get an effect from that medication for about 8 hours. I can give you an example: About 15 years ago, my son had an accident and he had to have his right leg amputated above the knee. The trauma of the accident had caused his kidneys to stop working, so he couldn't be given any IV or oral medication for pain relief. Instead, he was given a femoral nerve block and through the cannula he was given a slow, continuous infusion of local anesthetic. That relieved his pain effectively for 5 days, until he'd had renal dialysis and his kidneys started working again. Once it was safe for him to have oral medications again, the femoral block was removed and he was given a combination of Tramadol and Paracetamol (Tylenol). I'm sorry, but I don't understand what you mean by "I want to know what to ask for at the bar."