The participants took either caffeine (3 to 12 milligrams per kilogram of body weight) or a placebo, typically 60 minutes before exercise.

Compared to a placebo, caffeine produced small improvements in movement velocity and power output for the bench press and squat at loads of 25% to 90% of participants' one-repetition maximum. Taking a higher dose (more than 6 milligrams per kilogram of body weight) produced larger effects but also caused more side effects, including anxiety and increased heart rate. The greatest effect was observed in participants with low habitual caffeine intake (less than 3 milligrams per kilogram of body weight daily). The effects were also larger in men than women.

The evidence quality was rated as low to moderate, and all the studies showed some concerns related to risk of bias.

How much caffeine do you need to boost lifting performance? Original paper