Ask the doctor whether, among patients with the same first-year drug-use status, patients who dropped out would be more or less likely to have used the drug in the second year than those who remained. The comparison is within the observed first-year categories, not just between all dropouts and all completers.
If there is no remaining relationship with second-year drug use after conditioning on first-year use, missing at random is plausible for these recorded variables. If treatment-related difficulties, new medication or deterioration lead to dropout in a way not accounted for by the recorded first-year status, missing at random may fail. A different response rate in the two first-year categories is allowed under missing at random. The unobserved second-year outcomes mean that the observed table alone cannot establish this assumption; clinical knowledge or additional follow-up information is needed.
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