No you can't take it by yourself, you won't even be conscious long enough to save yourself. Also in most cases sn causes cardiac arrest short after, so the process of bringing you back is more than just methylene blue. It's best to make your mind before drinking it but sometimes things don't go as planned and who knows what the mind thinks in that state. So if you do change your mind after it's better to call for help asap before unconsciousness sets in.