If someone robs me and takes marijuana will I get in trouble?

It takes a bit of law to answer this question, and a bit of an ability to predict the future. First, let's get the offense down right.  If you or anyone else was present when he took your things, it was a robbery.  If he went into your abode when neither you nor anyone else was there, it was a burglary.   Not that it matters in the context of your question. The answer is yes, you can be charged, and possibly you will be, because one way he can lessen his sentence is by means of a plea bargain under the terms of which he tells the police where he got the controlled substance. Its not possible to tell whether the police will care enough about this to charge you.  If they do, if its your first offence, and if its truly an ounce or less, its only a misdemeanor.  If you've been found guilty of that offense previously, or if the quantity was more than an ounce, its a felony. But maybe he smoked it.  If he did, its gone and there's no evidence other than his testimony.  If the evidence has been smoked up, its highly unlikely they would charge you.