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If he has no prior criminal record then his best bet is to simply say that he truly did not know what it was until the police told him that it is a narcotic. He should tell the prosecutor that he believes his friend meant well and he (your son in law) does not take anything stronger than motrin and that is what he thought it was (prescription motrin). If he has no prior record then he may be able to get a break from the prosecutor -- what he should ask for is pre-trial probation/adjudication on the matter - the prosecutor has the power to continue the case without a finding at this time and set a review date for a few months down the line (usually the prosecutor chooses 6 months or a year) and if there are no further problems or issues during that time period the case will be dismissed and there will not be a "guilty" finding on his record. The benefit of not having a "guilty" finding on his record is that he can ask for the records to be sealed/expunged in a few years (they will not do it immediately) and then if the record is expunged he does not have to disclose the arrest to future employers. He will have to bring this up to the prosecutor (tell him to ask the prosecutor if there is a way to place the case on pre-trial adjudication and if he stays out of trouble during the designated time period the case can be dismissed against him? Tell him to tell the prosecutor that he needs to either not have a criminal record from this or, at the very least, he does not want a "guilty" finding on his record so he may be able to get the record sealed in a few years if he stays out of trouble). So long as he is pleasant and conciliatory and has no prior record, I am sure that he can get the entire thing taken care of tomorrow.
I hope that helps. Please let me know if you have further questions. If not, can you please press the 3rd, 4th or 5th smile face below so I will be paid for my time. Pressing a positive rating below will NOT cost you any additional money -- it simply acts as the trigger to Just Answer to pay me for my time. THANK YOU VERY MUCH