Can The Medicine Applied To The Anus Cause Side Effects?
I understand that you are talking about 3% hydrogen peroxide. What you have done is to prevent infection from the encounter that you have. At first place let me suggest you, that please avoid such risky practice as risk of getting serious infection is very high. If it is with regular and single partner it is ok. This is just a piece of advise as a doctor please pardon me if you not liked it.
Coming to your point if large quantity is used there are possibilities that if might have been absorbed in system,but looking to the site of application this seems unlikely.
In some cases large systemic dose can lead to diarrhea and vomiting. I think there is always a warning on it not to swallow. The anal area is very richly supplied with blood and anything can absorbed easily in the system from there. So be careful before applying any thing there.
Take more of fluids,light diet and rest. The diarrhea should stop on its own,if not then you need to take anti motility drugs under the guidance of your physician.
Presence of semen there when applied has nothing to do with this.local irritation is also possible due to this application and you can try some local anesthetic gel over there or even some mild cream can take care of this. Please use proper precautions next time so that you need to try such thing in future.
So take care and get well soon, still I will be happy answering your any more query if you have.