Tranq gun?
Tranq guns, when used on animals let alone humans have a wide range of problems, from that minor one of what pressure do you use? (too little and it won't go far enough and penetrate, too much and it'll potentially do serious damage).
The drugs have to be measured for the weight of the person (not something that can easily be done on scene in a hurry), too little and it won't work, too much and you've killed him...and there is the risk of any intolerances the person might have to your chosen drug (either natural, or because it reacts with something they've already taken - including legal substances).
Not to mention the risk of serious injury from the needle on the darts.
Also, I strongly suspect tranquilliser guns are restricted (I vaguely remember you need a licence for them), so would probably be something armed officers would carry, if they were a part of the police tool kit.