For some people, taking a course of antibiotics may cause digestive upset and diarrhea. That’s why so many people now take probiotic supplements after antibiotics. The probiotics will replenish the “friendly” bacteria that are killed by the antibiotics, restoring the “microbiome”—the balance of gut bacteria that we all need for good health (and good digestion).

But surprisingly, proof of the effectiveness of this strategy has been highly debated. Now two large studies have taken a closer look—and the argument for using over-the-counter probiotics after antibiotics does not look strong.

In the latest study, researchers treated 21 volunteers with a seven-day course of commonly prescribed wide-spectrum antibiotics. Some volunteers were randomly assigned to take an 11-strain probiotic mixture after the course of antibiotics. Result: The microbiomes of the volunteers who didn’t take probiotics recovered within three weeks of the cessation of antibiotics, while those in the probiotic group did not recover even five months after stopping antibiotics. 

While the generic, store-shelf-type probiotics didn’t help in this study, the future may hold an effective solution. In the same study, participants who received their own mixture of gut bacteria after finishing antibiotics—a procedure called autologous fecal microbiome transplantation—reconstituted their ­microbiomes within days of the procedure. This technique is not widely offered, but a time may come when doctors either routinely offer it to patients after antibiotics or can in some other way prescribe a probiotic formula that is precisely personalized to each patient…rather than using the current and ineffective one-size-fits-all approach.­