Sores in the mouth make it easier for some STIs to enter the body. For example, HIV is not commonly passed to the person giving oral sex. However, if there are open sores or bleeding gums, then the chances are a bit higher.
On the other hand, your partner would have to have an STI, in order to pass it on. The main ways that people get STIs is through sex. If your partner has not had sex with anyone else, then it is unlikely that he has an STI. In that case, you could have oral sex without worrying, even with a canker.
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