Ashwagandha root extract (600 mg/day standardized to >5% withanolides) administered for eight weeks in combination with resistance training is well-tolerated and demonstrates no adverse events in healthy adult subjects aged 18-45 years.

Study: Effects of Ashwagandha ( Withania somnifera) standardized root extract on physical endurance and VO 2max in healthy adults performing resistance training: An eight-week, prospective, randomized, double-blind, placebo-controlled study

The study gave exactly 600mg/day of Ashwagandha root extract (standardized to >5% withanolides) to healthy adults doing resistance training for eight weeks and found that nobody reported any side effects or adverse events - exactly matching what the claim says.