No, I haven't heard of anyone having a side effect to Serrapeptase, it's been a miracle cure for everyone I know that takes it which is my entire family. It saves lives! Maybe it was the brand you used, doctors best 120,000 has worked beautifully. It not only clears blood clots and dead tissue in the arteries, it dieloved mucus and is great for copd. I love Serrapeptase! My fav supplement! My 79 year old dad takes it every day with no issues.