For my son, it helps almost immediately after he takes it. Make sure you're using a good quality thiamine supplement, lipothiamine or allithiamine. Some will work better for people. My son takes the Cardiovascular Research brand lipothiamine but I know about parent whose teen takes allithiamine because it works better for them. You may want to try a few different brands of those 2 forms of thiamine if possible. Be careful when taking large amounts of thiamine, it can cause stomach pain and strong body odor.