The B1 and B2 require that you demonstrate non immigrant intent by exiting the country. Simply going to a border country and re entering does not demonstrate non immigrant intent. You risk the possibility that BP will cancel your visa. You need to go home to your country. if you still have time on your visa you may want to go to your local BP office and ask that the status be changed.
Based on your posting it does not sound like you have the correct visa for your circumstances. You may want to consider consulting with an experienced immigration attorney.
Assuming you may enter Canada or Mexico, yes, you can leave and re-enter. Make sure to let the admitting officer know the purpose of your entry so that you will have B-1 classification. I'm assuming that you are attending (or attended) med school outside the US and that you are entering for an observership or similar.
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That could work provided you present evidence supporting a B-1 admission.
J Charles Ferrari Eng & Nishimura 213.622.2255 The statement above is general in nature and does not constitute legal advice, as not all the facts are known. You should retain an attorney to review all the facts specific to your case in order to receive advise specific to your case. The statement above does not create an attorney/client relationship. Answers on Avvo can only be general ones, as specific answers would require knowledge of all the facts. As such, they may or may not apply to the question.