Hi, Yes, this combination is safe and routinely used in clinical practise. Infact the combination of invokana (canagliflozin) and metformin is available as invokanamet in market and it is an approved combination by FDA. Now coming to the side effects, looks like your husband is affected by the metformin component that has higher incidence of gastrointestinal upset. I would have recommended my patitent to take metformin immediately after food rather than empty stomach to reduce gastrointestinal symptoms. I hope you are aware that invokana has to be taken before breakfast in the morning. The side effects might have resulted in erratic blood glucose levels. The side effects of metformin usually resolve over time. Do ask for a change if you are feeling uncomfortable with the side effects. Have a regular follow up and maintain a well balanced diet with adequate physical activity. Hope the above information helps you. Thank you very much for writing into healthcaremagic.