I am taking penicillin because of an abssess I have. I notice the little acne popules on my forehead from rosacea are clearning up a little. But I started using the right cleanser and a little bit of BP. I'm wondering if it is the small amounts of BP or the anti bioltic penicillin?
What do you think?
Heck, I was prescribed pennicillin specifically for acne when my skin was absolutely wretched (I had started on a drug that worsens skin, was using an oily shampoo / conditioner and so I went to the derm). She started me with anti-biotics before stopping them and moving to topicals.
They work on acne, however, most responsible doctors will not keep you on it long term because it longer exposure could make bacteria develop a tolerance, meaning it may not work in the future (like during an infection, after surgery, etc).
i honestly wouldn't know. penecillin was a disaster to me. I"m allergic to it. Found out by waking up with a body wide rash that looked like i'd been sunburned through a vegetable strainer. It even spread to my face! then i swelled up and stopped breathing 
luckily i got to the hospital in time and they pumped me full of steroids. The rash vanished without a trace starting that day from my face and neck and finally left my chest and legs after 3 days.
Its not for everybody. I'm on minocycline which is a different family of antibiotics and it works. When i was a kid i was on oral erythromycin for awhile...(that was a mistake tho as now i'm immune to it and its the only thing to treat bronchitis so i woud never recommend going on oral erythromycin for any extended periods). Penecillin is so widespread tho i would never use it for acne...to treat acne you have to be on meds for an extended period. You should keep your options open if you get infections and never become immune to one drug like i am. I can still take topical erythromycin but i can't take the oral stuff anymore..this is disasterous as i get bronchitis alot (weak lungs) and as a result i get penumonia and am developing athsma.
Even tho penecillin might work...go on something else if you want to see results...don't narrow your options. Its just not worth dying over.
trust me. i know.
Penicillin is an antibiotic that targets the peptidoglycan in the cell wall of gram positive bacterium. Most acne causing bacteria are gram positive so it would only make sense that Penicillin reduces acne. Although taking Penicillin (or any antibiotic) when unnecessary can be extremely dangerous as it allows the bacteria to build up a resistance to the drug.
War Damn Microbiology