Slow-Mag Fizzies is a supplement taken when your body does not have enough magnesium. Magnesium is important to maintain muscle function and normal heart rhythm. They help to relieve muscle cramps, fatigue, and the effects of stress and improve recovery after exercise. 10 Effervescent tablets. Each tablet contains Magnesium carbonate equivalent to 172 mg elemental magnesium and Vitamin C 150 mg.