Omega-3 fatty acids, like drugs, work at therapeutic levels. For mono-polar depression that will be at least 5 grams of EPA and DHA per day, and for bipolar greater than 10 grams of EPA and DHA per day. The data is also clear that you need both omega-3 fatty acids, not just DHA. The high doses are required to get adequate levels into the brain to decrease the inflammation that is the underlying cause of depression because increased inflammation disrupts the transmission of neuotransmitter signaling.
Finding ways to make antidepressants more effective could help the millions of people who take them for depression. Now early research suggests pairing the medication with certain supplements, including omega-3 fish oil and vitamin D, could be one way to do so.