Good to know you are not a habitual drug user. Even better if you assured us you used it experimentally and have decided not to repeat. If you aspire to military service, random drug tests are part of the life, and penalties are severe.
To the best of my knowledge, you will not take a drug test for DoDMERB medical exam. But - you will be expected to respond honestly on medical history and security background check questionnaires on drug and alcohol usage. If you do not tell the truth on those, you will have to keep the lies straight every time you encounter those in military service. The military will not be surprised by young people who make impulse-driven or peer-influenced choices with drugs or alcohol in experimental amounts.
The DoDMERB website is dealing with an extended technical outage, or I would send you there to read about the process, FAQs, etc.
DoDMERB is the DoD agency which oversees the process to perform medical reviews for those wishing to enter military service. They find candidates either meet standards or do not meet standards, which is often referred to as Q and DQ. It is then up to the commissioning program waiver authority to decide to waiver a DQ and clear a candidate as medically eligible. The candidates has completed a medical history questionnaire and is sent to a contract provider to review the history and perform required exams.
And welcome new member of half an hour!