Yes, they can. There is one study that found diarrhea with high doses.
"DB Omega-3 Helps M-D in 4 Month Study: In a easy child DB 4 month study of 30 bipolar patients, 14 had fish oil added to their usual treatment. 64% on fish oil improved vs. 18% on placebos. Patients received 7 high omega-3 fish oil capsules (440 EPA 240 DHA) twice a day (9.6 g/d). Benefit appeared for both depressive and manic symptoms. Four patients treated with no psychiatric medications but fish oil did well vs. 3-4 on olive oil who did poorly. Stoll has used flax oil in open-label treatment. Starting dose=5g/d of omega-3 or alpha-linolenic with max dose to date 15 g/d. at over 10 g/d some diarrhea and oily stools. If patients take most fish oil HS, it decreases the aftertaste. Also, you can put fish oil in OJ. Perilla oil causes less GI distress. Most fish oil caps only 300 mg omega-3 so need 32 caps/d to equal study level. If flax seeds, 4 Â½ Tablespoons of 17.5 g flax oil = 9.6 g omega-3 equivalent. If flax oil caps, start with 10/d and 17.5/d=9.6 g. Stoll AL, Severus E, Freeman MP, et al. Omega 3 fatty acids in bipolar disorder: a preliminary double-blind, placebo-controlled trial. Arch Gen Psychiatry. 1999;56:407-412"
How about nausea? I just took 2 caps. today after breakfast and my stomach is flipping. Any idea? Gave it to my kids too. Hope they don't have problems in school! Can't handle all 4 of them coming home. EEKKKK!