Bitter melon extracts are thought to work in a very similar way as Amaryl (glimepiride). Both are thought to have mechanisms of action that stimulate the beta-cells of the pancreas as well as increase cellular sensitivity to insulin.
When introducing a natural product to address a health condition you are already taking a prescription medication for certain precautions need to be taken. In this situation I would not advise discontinuing your prescription medication because you do not know how the bitter melon extract will work for you.
Instead what I suggest is introducing the bitter melon extract at a very low dose while continuing on the glimepiride. Start a very low dose (1/2-1 capsule/tablet) per day for 7-10 days while increasing the number of times you monitor your blood sugar levels. If after 7-10 days you do not notice any significant declines in blood sugar levels, slightly increase the dose of the bitter melon and stay at that higher dose for another 7-10 days. Continue this process every 7-10 days, all the while closely monitoring your blood sugar levels. Also, make sure to always have a readily available source of sugar with you in case your blood sugar levels drop quickly or unexpectedly due to starting on the supplements.
If you do it in this way you give your body a chance a to adapt and get used to the new intervention slowly. It also allows you to track the trends in blood sugar over a longer period of time which reduces the chances that you will end up in a dangerous situation with blood sugar falling too low.
If over time as you build your dose of bitter melon you find that your blood sugar levels are hitting new low points, discuss this with your doctor. He/she should be able to adjust the dose of glimepiride so that your sugar levels do not fall so low. In this way you slowly reduce your dependence on the medication as you replace it with the bitter melon. You may find that you always need to take some of the prescription to manage your blood sugar or you may be able to come off of it entirely.
HOWEVER IT IS VERY IMPORTNAT TO DO THE PROCESS SLOWLY AND TO NOT ABRUPTLY DISONTINUE YOUR MEDICATION TO AVOID ANY DRAMATIC RISES OR FALLS IN BLOOD SUGAR.
Going about it in the way described above may take longer but offers you the best chance of long term success while minimizing the chances of causing any harm.