In an article published it in January issue of its magazine, it cites an investigation by sister publication Drug and Therapies Bulletin (DTB) which concluded that the evidence is "patchy" and more research is needed.
The majority of probiotic products on the market contain lactobacillus and bifidobacterium bacteria, but the precise effects of each closely-related strain of bacteria can vary wildly.
Moreover, probiotic bacteria must be treated very delicately in order to ensure they survive through manufacturing, transportation, storage and right up until they reach the gut.
DTB looked at 12 yoghurts, yoghurt drinks and fermented milk products on the market. It said that none gave the precise numbers of probiotics in the product, and the level of detail as to the identity of the bacteria varied. Which? believes that there should be some standardization in order for a product to be labelled 'probiotic'.
This is not the first time that questions have been raised about the use of the term - and not only from those outside the industry. In October, Kaarle Leporanta, head of marketing at Finland's Valio, said that too many producers are falsely labelling their products as probiotic. This, he warned, could present a stumbling block to the sector's expansion.
Leporanta called for more self-regulation in the industry so that the term probiotic is not used to promote so many products, and for strain-specific studies on health benefits.
Most probiotic products are marketed in the UK for general well-being, although in other markets the emphasis is placed more roundly on gut health. This is the area in which DTB was most convinced by the research available, but published studies into the effects for chronic allergies such as eczema were said to be limited and needing confirmation.
Last month Sweden's BioGaia pulled the plug on a two-year programme investigating the effects of probiotics on allergies following disappointing results.
Which? said that probiotics are safe for most people to try, but advised those with a weak immune system to speak to their doctor first.
Eight of the 12 product labels looked at said they contained more than 10g or sugar per 100g - a level that the Food Standards Agency considers to be a lot. However manufacturers have defended the need to sweeten products to make them more palatable, since fermented dairy products tend to have a sour taste.