Neomycin (antibiotic) interferes with Vitamin B6 absorption. An error in the uptake of B6 can cause a rare form of epilepsy and mental retardation.

To reduce the development of drug-resistant bacteria and maintain the effectiveness of neomycin sulfate tablets and other antibacterial drugs, neomycin sulfate tablets should be used only to treat or prevent infections that are proven or strongly suspected to be caused by susceptible bacteria

Neomycin product info for oral use:
SYSTEMIC ABSORPTION OF NEOMYCIN OCCURS FOLLOWING ORAL ADMINISTRATION AND TOXIC REACTIONS MAY OCCUR. Patients treated with neomycin should be under close clinical observation because of the potential toxicity associated with their use. NEUROTOXICITY (INCLUDING OTOTOXICITY) AND NEPHROTOXICITY FOLLOWING THE ORAL USE OF NEOMYCIN SULFATE HAVE BEEN REPORTED. Can result in neuromuscular blockage and respiratory paralysis have been reported following the oral use of neomycin.

As with other antibiotics, use of oral neomycin may result in overgrowth of nonsusceptible organisms, particularly fungi.

Source: Dailymed