What happens if you do not take the preventative medicines before you go to the dentist?

You may die: If you do not take the antibiotic prior to going to the dentist, you may get an infection called bacterial endocarditis which often leads to death. If the drug is due to an artificial joint, you may get an infection in the artificial joint and have to undergo a replacement surgery.

Nothing probably: If you are speaking of antibiotic prophylaxis for heart or other issues, likely nothing. The use of antibiotic prophylaxis has been going out of favor in recent years, mainly due to the fear of allergic reaction with antibiotics. You should take your antibiotic premeds as prescribed, but if you missed them for one appointment, it shouldn't be a problem. The aha has reduced the use of antibiotics.

Nothing: Unless you needed it, it is not required.

Maybe bad things: If you need antibiotics before dental procedures and fail to take them you could get a serious infection. If you need sedatives because of epilepsy, failure to drake them could lead to a seizure. If you are extremely anxious and react violently to a dental procedure, you could be injured. On the other hand if you're healthy and not anxious you don't need anything. Hope this helps.

Which medications ?: It would be great if we knew which medications you were talking about and what procedures were being contemplated. Please repost with more specific information.