Both are antibacterial remedies. Tetracycline is an antibiotic. It's quite commonly available, but it's a bit outdated today as many of the bacterial strains are resistant to it already. Doxycycline is better, but need prescription to get them.

I believe Furan treats gram negative bacterial problems and tetracyline is a broad antibiotic which treats both gram negative and positive bacteria. Me no doctor so I may be wrong....