I started taking 2 grams of MSM per day for about 9 days (NOW Foods brand in capsule form). As soon as I swallowed a capsule, I became excessively thirsty. I'd even wake up in the middle of the night to drink 2 full glasses of water. I perspired much more (perhaps from all the water I drank), which didn't bother me because I liked sweating more in the sauna. I got pains in my body, especially in my arms and hands - I felt like I was getting arthritis/tendonitis, which was weird because I thought MSM was supposed to help if you have that. My bowel movements increased from 1 per day to 4. At night I'd notice my heart was beating fast. I can usually walk for miles but I got really scared when I found I had trouble walking fast and breathing well on a fairly short walk. I was searching the web for allergic reactions to MSM but couldn't find any. The only other supplement I was taking at that time was 50mg of Ubiquinol (CoQ10). So I stopped taking both. The thirst and breathing/heart problems went away fairly fast, but the pains in my arms lasted a few weeks. Since the time I stopped taking it (probably 1.5 months now), I've been getting sharp pains in my upper abdomen (left and right sides), which have been slowly getting better.