@wonderer You'll be perceived as weak if you rarely or never stand your ground, but it will not weaken you to agree once in a while. Most likely you'll be perceived as much weaker if you can't stop with drugs, if others perceptions about your strength matters to you.
Definitely do not lie, dishonesty is a real evil. So this is a two way choice if you're going to play it fair, but it is YOUR choice. You know the possible risks and consequences of your two options. If you quit doing it and agree you'll lose the fun you have on drugs once or twice a month, but you'll get her and will probably find other things to enjoy. If you don't quit you risk losing her, but that's not a certainty. She might eventually accept it anyways because she faced her own choice, to be with you despite this or to lose you.
You need to assess for yourself what's most valuable to YOU. Is it doing recreational drugs once in a while or is it this girl? You must know or figure out what you want the most, the forum cannot possibly know this and everyone who answers does so out of their personal preference.
Don't make a choice based on your image of being weak or strong, that's just stupid. Choose based on what direction you'd like going in life. None of us can have it all, sometimes we'll lose some no matter what we choose. In such a situation it's about reducing the loss and choosing what's most valuable to you, but again we can't define to you what's worth more.
Just out of curiosity, what drug(s) are you taking?