Concerned about Medical test

I am well aware of the strict policies in Saudi Arabia in relation to drugs etc, I am about to commence a full medical as part of the recruitment process, I have recently been to Amsterdam for my cousins stag do and although I didn't personally smoke any drugs I'm pretty sure I would have absorbed some passively and I am very concerned about failing any drug tests involved in the medical examination. Does anyone have any experience or advice for me as this job is something I have wanting to do for years and do not want to jeopardise this opportunity, can anyone help me please.
K.