If you're a dealer- probably. If you're some normal person texting about drugs- probably not so bad.
Generally it's always a bad idea to have stuff about drugs written in plain english in your pocket though. You wouldn't write "Dear john doe: hey I just got some heroin, do you want to buy some?" on a piece of paper, fold it up and stick it in your pocket would you? All someone has to do is open up your phone, hit one button and boom there it it. It's even dated and timed and has list of who you sent it to. This includes cops, nosy girlfriends, anyone else you wouldn't want to see you talking about drugs.
ALSO:
This can be used for entrapment purposes.... I'm not so sure wether it's legal or not but I know it's happened more than once where I'm from. Someone will get arrested and the cops will take their phone and set up a phony deal with your dealer and arrest them, all because you didn't delete your texts. I've straight upseen cars get rushed by multiple cops in a parking lot for unexplained reasons. That's real good for your name I'm sure your dealer will be thrilled with you.