Some drugs definately can induce a state of euphoria and cause a person to smile constantly. However, no drug can overcome pain and discomfort while still allowing the subject to look genuinely happy. Opiates are the only drugs that truely disrupt one's pain recognition, but under doses strong enough to do this, the subject is ALWAYS 'out of it' because of the nature of opiates. Simply put, there are no drugs that can make a person act happy when they are undergoing severe discomfort.

Please don't ask me how I know this because I will not declare that I have broken the law on this forum. Trust me though, there is no such drug and anybody with experience using psychoactive substances will tell you the same. True pleasure is nearly impossible to fake, with drugs or without. The claim that children seen to be enjoying themselves during sexual activity are no doubt under the influence of drugs is either ignorant or otherwise motivated. Drugs will never change a person's perceptions enough that they won't object to pain unless given doses great enough to completely dissociate that person from reallity.