All sweetner is bad for you whether used in tea, coffee, diet drinks etc. Basically when it hits your tongue your body expects sugar so releases a load insulin to break it down. When it realises you haven't eaten a load of sugar it suddenly has a massive dose of insulin floating round your system that it has to do something with. Have too many sweetners a day for too long and your blood sugar system can get really messed up contributing to diabetes. In small doses they're not too bad though.
Thanks girls how scary. I will try stevia. I don't use loads but have 3 in black coffee and one in my tea. I might try going without. Don't want to drink too much black coffee and tea anyway. I must have a cup of tea in the morning though. I see if I can do without. Thanks for the info. Glad I asked that question cuz I was just gunna ignore it.