Some general side-effects that usually occur are dizziness, diarrhea, flushing, headache, heartburn, upset stomach or even stuffy nose. However, there are some more serious effects that it can have on your body.
Regarding your vascular system and heart, it can cause the rhythm of you heart to increase, which is usually followed by heavy pounding. And if you had any alcohol previous to the intercourse, the feeling gets stronger. So, in case you notice that your heart beats unusually fast in moments like this, you should stop and relax, and let your body calm down, for if you continue straining your body and your vascular system, consequences can eve be fatal.
As regards your nervous system, some slightly serious side-effects can be depression, decreased reflexes, sleeplessness, anxiety, loss of sensors… However, temporary memory loss ad seizures are the most severe ones. You might want to inform your partner about this, in order that they can help you in a proper way and that they be less distressed.
Vision abnormalities. Specially some things will take a blue hue to them or you will not be able to tell what's green or blue. Some men could experience priapism (an e*rection that doesn't go away and last more than four hours) Some men with an undiagnosed heart condition may experience low blood pressure and heart attacks.
Very congested stuffy nose, headache, those are common side effects.