Recent FDA Guidance, currently still a draft, aims to clarify the way that the quantity of live micro-organisms present in food supplements can be declared. At present, the amount of all dietary ingredients present in dietary supplements must be indicated by weight per serving. However in the case of probiotics, this approach is not an adequate method because the weight does not reflect the number of viable microorganisms in the product. The presence of live micro-organisms is preferably indicated using Colony Forming Units (CFUs). For that reason, the FDA has considered the use of quantitative indications different from the declaration by weight for use within the supplement facts label. Currently, the label may bear information on the amount of living cells, but only outside the supplements facts section. To allow otherwise would in essence require a revision of the Supplement Facts Regulations. However, the Guidance clarifies that a discretionary approach is applied - allowing for the additional indication by CFU - when conditions as listed in the Guidance are respected. It is stressed the flexible approach only applies for dietary supplements containing probiotics but not for any other dietary ingredients or other types of foods. The current Guidance concerns a draft version, meaning comments can be forwarded until 6 November 2018.
US FDA - Draft Guidance for Industry: Policy Regarding Quantitative Labeling of Dietary Supplements Containing Live Microbials - link