'Modest' just means you don't require a helper to perform every day tasks. It can be be very debilitating, but isn't 'Major' until you can't perform every day activities on your own. For further context, Trisomy 21 (Down Syndrome) is characterized as 'mild to moderate'.
Modest: No interference with independence in everyday activities, although these activities may require more time and effort, accommodation, or compensatory strategies
Greater context, which doesn't seem to line up well with your assertion here:
> ...that higher doses and heavier use of cannabis are associated with modest negative effects. These findings are consistent with other systematic reviews and meta-analyses showing similar small adverse effects on cognition in younger populations. With that said, there was also evidence of modest improvements in subjective cognition on patient-reported outcome measures, particularly for medical cannabis users.
Because of a subjective improvement? I don't follow what you mean. People believe they are subjectively driving fine while hitting every mailbox due to being drunk.
