What Causes Constipation After Taking Tinidazole?

My doctor has prescribed tinidazole for two days yesterday (thursday) and today (friday) for an upset bowel. I told him i would go out and have a few drinks on saturday night(tomorrow) and he said that would be fine but now i read that alcohol should be avoided for three days after treatment. What are the chances that if i drink the night after two days of treatment i will have an interaction? Also i was going to try a weed brownie tonight as i was avoiding alcohol, are there any interactions between tinidazole and marijuana?