Hello Welcome in H.C.M. I have gone through your query regarding spitting of brown phlegm when you are taking antibiotic for sore throat and cold. As you said that you were having cold and sore throat and for this you are being given antibiotic penicillin, now your throat is better as there is no pain but today you had spitted a brown phlegm. This brown phlegm is due to old exudated blood and seru from diseased mucosa of nose and throat. Need not to worry as it is brown not fresh blood. Since there is no pain on degulitition it means your infection of nose and throat is resolving. I hope above explains your query. Regards.