Gut flora partially recovers on its own after antibiotics, but almost never completely. After a short course, some bacterial species disappear for months or longer; after repeated courses the composition can change permanently. Probiotics demonstrably delay the recovery of one's own flora and are therefore not a good means of microbiome restoration, even if they can reduce diarrhoea as a side effect. All of this has been established in small human studies, so it should be regarded as compelling evidence, not as a definitive conclusion for everyone.
Antibiotics are one of the most disruptive factors for gut flora, alongside age, mode of birth, and diet. A broad-spectrum course throws the composition of gut bacteria into considerable disarray. This is well documented in multiple studies. The question is not whether you are affected, but how long recovery takes and how complete it is.
When it comes to recovery, the picture is mixed. After a short course of three broad-spectrum antibiotics (meropenem, gentamicin, vancomycin, 4 days), the flora of healthy young men largely recovered within about six weeks. However, nine bacterial species that had been present in all twelve participants before the course were no longer detected even after six months. There is therefore a mild but lasting mark left by the antibiotics, even after a short course.
Repeated or longer courses are more problematic. A small study using ciprofloxacin (three participants, two consecutive courses) showed that diversity dropped sharply within three to four days. After stopping, the flora did begin to recover, but in all three participants the composition was still different from before the course after ten months. This suggests that repeated courses can push gut flora into a different stable state. This is a small study; the finding is indicative, not definitive.
Probiotics after a course of antibiotics seem intuitively like a good idea, but the research points to the opposite when it comes to restoring one's own flora. In an Israeli study, probiotics containing Lactobacillus bacteria actually delayed the return of native gut bacteria, both in gut tissue and in stool samples. Recovery remained incomplete throughout the entire study period compared with spontaneous recovery. Some clinical studies do suggest that probiotics can reduce antibiotic-associated diarrhoea, but it has not been shown that this works via restoration of the microbiome.
The most effective strategy for rapid recovery to emerge from the studies is autologous faecal microbiome transplantation: people received their own previously frozen stool back after the antibiotic course. This allowed gut flora to recover within days, almost completely, and considerably faster and more fully than through spontaneous recovery or probiotics. This is currently a research method and not an available treatment, but it shows that one's own flora can in principle return quickly when the right starting bacteria are available.
Claims are based on: one RCT/controlled study with 12 participants (PMID 30349083), one observational study with 3 participants (PMID 20847294), one controlled human study on probiotics and aFMT (PMID 30193113), one review (PMID 32398103), and one clinical dataset on side effects (PMID 35045345). Participant numbers are small; the findings are indicative but not definitive at the population level.