How long does it take to get the trace of weed out of a system? my boyfriend was on blood thinners because of a heart condition he has and he smoked pot with an old friend of his and when he had to take a drug test it showed up positive. he failed to tell

If : If he was a heavy, regular user of cannabis than a urine drug screen may pick it up a month later. However, if used little of it and it wasn't being used regularly, it is usually out within a couple of weeks.

A few days: If it is truly only one use it will show up for 3-4 days at most, depending on the amount smoked. However, if it is regular smoking of marijuana, it can last many weeks or even months. If your husband is on blood thinners, he needs to know that there are a huge number of potential drug interactions that can increase or decrease effectiveness of blood thinners and put him at risk, including pot.