Brief Answer:
Yes, it's safe but make sure they know about all drugs you use.

Detailed Answer:

Hi,

ECT is very effective treatment options in many psychiatric diseases. However, because of its serious nature it should only be used if very necessary i.e., either a rapid cintro is needed or drugs have failed to work.
There is no problems reported in using ECT in patients using marijuana; rather this has been used in patients who have severe mental disorders secondary to marijuana use (like psychosis) who do no respond to medial therapy .So don't worry about it .

However, as ECT induces a brief seizures so the doctor must know any drug you are taking. (so they may give you a different dose of the ECT if necessary)
Generally we take a detailed history about all the drugs in such patients and a list is in hand of any drug being used by the patient. So I understand that this should have been asked by the doctor, but it's fine you can always tell them about this. So let them know about this (and anything else you understand necessary) the day you see them. (may be they ask u again once it's your day of ECT).

Regards

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