Answered by Shaykh Faraz Rabbani
Smoking marijuana and prayer validity
The sinfulness of smoking marijuana does not affect the validity of one’s prayers. Rather, one should be hopeful that the latter will be a means for one to overcome the former, for Allah says, “Verily, prayer prevents lewdness and wrong.” [Qur’an] At the same time, one should fear that one’s sins may weigh one down such that Allah divest one of the honor of being able to submit to Him in worship.
The way forward is to thank Allah for being conscious of the gravity of sin; to sincerely repent to Allah; and to take the effective means to change the course of one’s life. Keeping good company and busying oneself with good deeds are very strong means of being successful in one’s repentance, and shunning bad company and the situations and places that lead one to sin are key to remaining successful.
And Allah alone gives success in truly turning to Him.