Instant glue is great for holding many things together, but you should never use it on your dental crown. The gluing may not work, and it may even be dangerous for your health. Here are specific reasons not to reattach your crown with instant glue:
Cyanoacrylate Is Poisonous
Using instant glue for replacing a dental crown is also dangerous because cyanoacrylate is poisonous. There is a risk that the chemical may get into your tooth's pores and reach its roots. If that happens, you may end up with a dead tooth or one that requires a root canal procedure to save. Not only that, but you may also be poisoned if you accidentally swallow some of the glue if it spills in your mouth, and that has negative health consequences too.
It May Glue Your Body Parts Together
Since instant glue exists in liquid form, there is always a risk that some of it may accidentally spill on unintended parts of the body. For example, a drop of the glue on your lips may end up sticking your upper and lower lips together. This may happen, for example, if something startles you during the application.
It Takes Multiple Factors to Keep the Crown in Place
Lastly, you should know that the dental cement or bonding material isn't the only thing holding your crown in place. It follows, therefore, that a weak bond isn't the only reason it may fall off. In such a case, merely gluing back the fallen crown will not solve matters. For example, a crown may fall off it the tooth structure it can "grab on to" is insufficient. This may happen due to decay or damage incurred during tooth preparation before fitting the crown.
You May Glue Back the Crown Incorrectly
Lastly, even if the instant glue works, there is always the danger that you may fix the crown incorrectly. Since you aren't a trained dentist, you may not know the correct way to position the crown on your tooth. An ill-fitting dental crown can cause problems. For example, it may rub against and erode an adjacent tooth, cause eating difficulties, or allow bacteria to grow underneath it.
In short, resist the temptation to use instant glue to reattach a fallen dental crown. The best thing you can do if your crown falls off is to see the dentist. If you can't do that right away, then get hold of some temporary crown cement and replace the crown until you can see the dentist.