Do Antibiotics affect performance?

I am currently on AB in order to kill off the Helicobacter Pylori, that thing in the stomach which can cause ulcers and gastritis.

Was more or less off the bike for 4 weeks with some easy commuting (300km) when I started the treatment consisting of 2 different AB and 2x6 pills of them (plus another two meds. So a total of 18 pills a day.).

It is safe to assume that I have no infection slowing me down. Don‘t know how mich the FTP can drop, but now I am at start of SSB2 5% below my first testresult upon starting SSB1 in Nov.

The trouble are not the legs, but that my body is really struggling much more to absorb oxygen.