A survey for the watchdogs suggested less than half of people look for dosage instructions on supplements, with 20% saying they did not read the label.
Research also found that many consumers have limited or no knowledge of caffeine in food supplements.
Caffeine is used by some gym-goers, with some fitness websites recommending it for improving sports performance in certain doses.
FSA and FSS guidance says supplements with caffeine should be calculated alongside other sources of caffeine, such as coffee, tea and energy drinks.
Caffeine intakes of up to 400mg per day are unlikely to cause adverse effects in adults, but pregnant women are advised to limit their consumption to 200mg.
