Most temp. tattoo's use Henna which is perfectly safe in it's primary state. The true colour of henna is a dark green colour. Any other colour and it's not henna!!!, no matter what they tell you henna only comes in one colour. So if it's black or brown trust me it has some type of additive added changing the colour or make it last longer. Henna only lasts for 7days before it goes off. The reason one day you're fine and the next day you're not depends on your hormone levels at the time the tattoo was done and hormone levels can change each day depending on many factors normally this is unknown to the person until they put chemicals into their bloodstream the body can't cope with and then you get the reaction. Yes anything that is applied onto the skin will make it's way into your bloodstream. So is it really worth it??? Buy the tattoo's you apply with a damp cloth and peel away the backing, much safer. I am not a doctor, just a mum who wants to know what's in things I use... So if you don't agree with me that's fine too.