yes you are allowed pain killers and no you cant have the shakes in one go, you have to split them up so your body gets what its needs in doses throughout the day. there are other posts on here regards doing different stuff with your shakes, have you tried the bars and soups, you can add teas to the shakes including peppermint etc or make them with less water to make mousse, this one might work for you if theres less of it but you still have to spread them out like your water, you cant drink 4litres in one go! i only like the vanilla and i gag with that so i add lots of ice so it strips the flavour a bit being so cold, the soup i cant eat any more but i used to, i used to put spices in it to cover the flavour of that too but cant stomach that one anyone. just keep going and persist with it, if it was easier non of use would be fat! x