The laparoscopic Roux-en-Y gastric bypass (LRYGBP) is increasingly popular for the treatment of morbid obesity, although its postsurgical complications are often underestimated. We report the case of a 22-year-old morbidly obese woman who underwent a LRYGBP, which was rapidly complicated by portal venous thrombosis and severe neurological complications due to vitamin deficiencies. She presented rapid body weight loss with optic and peripheral neuropathy. Clinical chemistry results showed low transthyretin and micronutrient levels. Intravenous micronutrient infusion and cyclic nocturnal enteral tube feeding were needed to slowly improve gait and visual acuity. We then discuss (1) factors that could have contributed to the nutritional deficiencies and (2) the preventive management of these types of nutritional complications. Gastric bypass procedures can cause multivitamin deficiencies. In the case presented here, complications occurred very soon after surgery. The increasing incidence of obesity and bariatric surgery warrants better patient education concerning strict adherence to vitamin supplementation.