Aminoglycosides are antibiotics which do a lot of damage to good bacteria colonies in the gut. If they are prescribed over a long time, they will completely destroy the good bacteria under their attack, leaving bad bacteria free to reign.
Combinations of antibiotics such as these are more dangerous than single ones. They leave the gut full of empty pockets for pathogenic bacteria to install themselves in.
Penicillins let bacteria found usually only in the bowel to move into the intestines, predisposing you digestive problems such as IBS (Irritable Bowel Syndrome).
Tetracyclines are often given to teenagers often over the period of a few years. They change protein structures in mucous membranes making the gut wall vulnerable to the invasion of dangerous microbes whilst informing the immune system that it needs to attack these proteins. So these antibiotics are capable of initiating an auto-immune reaction in the body.
Even when the antibiotics have been prescribed for a short time, it takes beneficial bacteria a long time to recover. Time to fix your gut?