While there's no definitive tolerable upper limit for HMB, the International Society of Sports Nutrition has found 3g/day to be generally safe and effective, particularly among untrained users.Wilson, J. M., Fitschen, P. J., Campbell, B., Wilson, G. J., Zanchi, N., Taylor, L., Wilborn, C., Kalman, D. S., Stout, J. R., Hoffman, J. R., Ziegenfuss, T. N., Lopez, H. L., Kreider, R. B., Smith-Ryan, A. E., & Antonio, J. (2013). International Society of Sports Nutrition position stand: Beta-hydroxy-beta-methylbutyrate (HMB). Journal of the International Society of Sports Nutrition, 10, 6. Life Extension Wellness Code, our winner for best overall HMB supplement, delivers exactly that amount per serving. Our other recommended brands range from around 1,000mg (Bulk Supplements and Double Wood Supplements) to 1.5g (Transparent Labs) per serving. Some studies have administered up to 6g/day with no serious adverse effects, but the available research doesn't support any greater efficacy with the higher dose.