Mixed

Caffeine doses of 4-6 mg/kg enhance upper-body strength (bench press 1RM), but lower doses (2 mg/kg) do not, showing a linear dose-response trend.

For upper-body strength (bench press), low doses of caffeine (2 mg/kg) are ineffective. You need higher doses (4-6 mg/kg) to see a statistically significant, albeit trivial, increase in weight lifted. However, the practical benefit is likely negligible for most non-competitive lifters.