I m in the process of transitioning from 150 mg Effexor to 20 mg Brintellix. During the first week, I withstood the telltale Effexor withdrawal symptoms: waves of uncomfortable wooziness and feeling light-headed. But 10 days into the transition, I began experiencing flu-like symptoms: headache, muscle aches (all over), weakness, fatigue and a cough. What would be the likely cause of these more debilitating symptoms - Effexor withdrawal or Brintellix side effects?