Magnesium sulfate is a compound containing magnesium, sulfur and oxygen, with the formula MgSO4. It is a possible abortive treatment for migraines.
1g intravenously over 15 minutes.
Possible Side Effects
One small study involved the abortive treatment of a migraine by administering 1g of magnesium sulfate over 15 minutes. 100% of the patients showed improvement, in either pain, or other symptoms. 87% of patients ultimately experienced lessening of pain. However, 86.6% also reported mild side effects to the treatment. 
- Mauskop A, Altura BT, Cracco RQ, Altura BM."Intravenous magnesium sulfate rapidly alleviates headaches of various types." Headache. 1996 Mar;36(3):154-60. https://www.ncbi.nlm.nih.gov/pubmed/8984087 February 23, 2017.
- Schnakenberg, Ellen. "Migraine Treatment: IV Magnesium Sulfate." Migraine.com. May 15, 2013. https://migraine.com/blog/migraine-treatment-iv-magnesium-sulfate/ February 23, 2017
- "Efficacy of intravenous magnesium sulfate in the treatment of acute migraine attacks." Gülhane Military Medical Academy Neurology Department, Ankara, Turkey. 2001. http://www.ncbi.nlm.nih.gov/pubmed/11251702 February 23, 2017