I have never read anything about doxycyclin making you fat, and weight gain is not one of the listed side-effects. Spironolactone is a medication that helps hormonal acne that some people have reported makes them gain weight, but I lost weight while I was taking it (about 12 pounds). As far as I know, there aren't any antibiotics that make you gain weight. I certainly didn't gain weight while taking amoxycillin, bactrim or doxycyclin.

And of course - topicals are not going to make you gain weight either.

Most medications will probably require a long term or repeated dose.