Zane, there are a few different types of medications that are prescribed for myotonia. The most effective seems to be mexiletine (brand name Mexitil). It is normally used to treat heart rhythm problems, but by blocking the function of the sodium ion channel it also helps with myotonia. You will need to get a thorough cardiac workup (EKG, ultrasound) and started on a low dose which is gradually increased. You will get monitored every 6 months or so. For most the only side effect is stomach upset which can largely be prevented by taking the capsule with food or with an antacid (never take on an empty stomach).
Carbamazepine (Tegretol) is often prescribed if Mexitil is not available or tolerated. I am very concerned about the use of this medication for anyone under 30. It can have some serious psychiatric side effects including severe depression and suicidal thoughts. It is usually prescribed to treat epilepsy.
Acetazolamide (Diamox) is effective for many people and has a better safety profile. The main side effect is ringing in the ears and a distorted sense of taste at first (especially when drinking carbonated beverages). There have been some reports of permanent hearing loss over time with high doses. This is called a carbonic anhydrase inhibitor and acts like a diuretic to increase potassium loss through the urine.
Quinine Sulfate is probably the oldest treatment for MC. It was used to treat malaria and leg cramps. It also has some side effects related to hearing.
Lamotrigene (Lamictal) is another anti-convulsant drug that can be used to treat myotonia. It is a mood stabilizer and is often a good choice for anyone with MC who also has bipolar disorder.
Flecainide (Flecaine) is another anti-arrhythmic cardiac drug like mexiletine, but it is not quite as effective according to people who have tried it for MC. However it is often prescribed in areas where mexiletine is not available.
All of the medications prescribed for MC have potentially serious side effects so they need to be monitored and the dose adjusted up gradually. Based on the feedback I have gotten and the safety profiles, I would personally choose mexiletine first, flecainide second and acetazolamide third.
Many of us don't use any medications and adjust our diet and lifestyle to minimize the myotonia. There are several supplements that can help myotonia. The most effective is the amino acid L-Taurine. This was shown to reduce myotonia in studies done in Italy. I have known several people who took it with good results. The dose would be 1-4 grams a day. It is considered very safe.
I use two supplements that are quite effective. One is Acetyl-L-Carnitine which helps with muscle function, and the other is Licorice Root Extract which increases potassium excretion. The carnitine has no contra-indications that I know of, and the licorice root is only contraindicated for someone with high blood pressure. I take one 500 mg capsule of each per day.
Capsaicin is also an effective treatment for sudden or severe myotonia. It stimulates the chloride ion channel and can relax the muscles immediately. I carry red pepper flakes with me (the kind you put on pizza) and chew on a couple if I'm having really severe stiffness or trouble breathing because my diaphragm is so stiff. This is especially good to use if you have been exposed to pesticides or have had a sudden reaction due to being startled. Hot peppers, hot sauce and salsa will have a similar effect.
Most of the medications will be effective very rapidly. Some of the teens on the forum use Mexitil as needed to play sports, but don't take it on a daily basis. That's something you would need to discuss with your doctor. In general, the less you can get by with, the less tolerance you will develop and the longer you will be able to use the medication effectively.