Whether they understand or approve is for themselves to decide, hiding your use is dishonest and devalues the relationship as you non-consentually dismiss or disrespect their opinions, feelings and thoughts, leading to an unequal relationship. I can relate somehow as my own substance use is sometimes connected with shame from self-stigmatisation, but I believe in a healthy and fair relationship these struggles can be overcome.
Of course you can be free to use whatever you want to use whenever you want to, but the outcomes of drug use (especially illicit!) can be severe enough that you should give your partner the ability to consent whether they want to be together with you or not. That's just my opinion though.