Drug detector and incomplete knowledge.

Is there any side effects of drug detector and my main problem is that I am confused about the action of medicine in our body I think may be it is due my lack of information about medicines so I want to know that is there any medicine which starts it's action after long no. Of days i.e. 10 or 20 and after this it acts very slowly that we even don't know that is it working or not actually I was frightened by thinking that if this would possible then anybody can destroy any person very easily without any guilt on him/her like poison can be given we will even don't know after a long time it works slowly I am asking about onnly a single dose can a single dose of any medicine can do this please halp me and I also want to know what are the chances in future of these type of medicine