I have swallowed many times and never had it make me sick. Perhaps if you are doing it on an empty stomach it's could bother you? I have heard of people getting sick but it's only out of a surprise and gagging during but never nausea after. Maybe it's psychosomatic?

There is nothing in a healthy man's ejaculate that should make you sick. Sera suggests psychosomatic and that is likely the case. The taste and consisteny have gagged many a woman over the years and the cheek walls are very similar to the vaginal walls. Therefore, they will abosrob a bit of the semen and keep refreshing the taste in your mouth. I always have the tasste when I wake up in the morning following this activity.

If he had eaten some kind of food that bothers you in the last several hours that could contribute. Like Sera, I have never been sick.