I have only done two flushes and my husband did his first last weekend.
He had a really bad headache, stiff neck and nausea throughout the night. The magnitude of all this decreased with each BM and he felt fine by about 10am.The first and second Epsom Salts in the morning really moved this process along for him.
I flet about queezy heading into each BM on my second flush, but generally fine.