I'm 33 and have had moderate to severe acne my whole life. It didn't finally start to get better til my late 20's, and has been better for a while on my face but is still not going away on my back.
I was on minocycline as a teen and my dr recently put me back on that but I couldn't stand it. I know that everyone is different, but I seem to be the person in the minority that experiences the rare side effects of most medications. Yay me. So anyway I took the minocycline for 5 weeks but had to stop because it gives me this weird fuzzy/foggy/light headed feeling all day. I equate it to the feeling some people, including me, get from taking dayquil for instance. It made it very hard to concentrate at work, so I started taking it at night....only to find out it made it very hard to sleep! I thought I had to be imagining this, but I researched and found many instances of people talking about getting insomnia from minocycline or anxiety/concentration issues. It seems rare, but I guess some people get that from it.
Well I told my dr and he gave me keflex (cephalexin) instead. I researched that before my first dose and was happy to see no side effects reported other than diarrhea. But what do you know, I take it and I get the exact same reaction that I get on minocycline. It's like if I take it in the morning then I have this weird fuzzy/anxious/on edge feeling all day and towards the end of the day I can kind of feel it wearing off and I start to feel more normal. I tried skipping a few days to test it out and I did feel much more normal on the days I didn't take it.
I can't find anything saying these are side effects of keflex, I actually saw many positive reviews of the drug on this site with basically no cons reported....so I am wondering...has anyone ever had issues like this with keflex or minocycline for that matter? Could I be "allergic" to these types of antibiotics? If I was though I wouldnt expect mental side effects vs physical ones. I havent had any issues with diarhhea or upset stomach, dizziness, etc. If anyone's got any insight it would be much appreciated.
I just got off of Keflex also for a infected lymph node. The only side effect I had was diarrhea the first couple of days, which my GP said would probably happen. There were 3 warnings on the bottle telling me it would cause it too. Even one that said it could cause severe diarrhea months down the road. Weird. I also was no aware they prescribed this medication for acne.. Are you taking it as a antibiotic for acne or a antibiotic for a infection?
I had to share this incident with you; hopefully it can be of some benefit.
Like most all drugs, side effects are possible, with some people.
I was given this drug for a possible sinus infection.
I woke up in the hospital, at about 2.00 AM one early morning, and was unable to think. I could not think of words to use. I did not know where and why I was in the hospital. I was seriously confused. I got into an arguement with my sister who was there.
I went to sleep and when I woke up I asked the kind nurse about the anti-biotics that I was given. She gave me a copy from Woleters Kluwer Health.
Under side effects it includes: WARNINGS.
Feeling very tired or weak.
Mood changes
Change in thinking clarly and with logic. *******
Hallucinations.
...(there is more...)
Not only this I woke up with a headache.
Thanks to the nurse for giving this information in the hospital.
Hope this helps. 
Caesar J. B. Squitti H.B. Comm
Author of the book, "The Jesus Christ Code".