I'm sorry I can't comment on specific oral drugs, but I do want to say you might consider your father being seen by an endocrinologist. A blood sugar around 155 is too high and after 10 years it might be time for him to go on insulin to prevent further complications.
It is time to control the diabetes by natural means and try to reduce the medication.
These pranayam exercises will help control the diabetes and the side effects.Build up the timing gradually.If you feel tired or dizzy, stop and resume later(after about a minute).The benefits will be noticed in weeks as the sugar level is checked daily.Over the long term the diabetes will be in full control.
Kapalbhati -(Do it before eating) Push air forcefully out through the nose about once per second. Stomach will itself go in(contract in). The breathing in(through the nose) will happen automatically. Establish a rhythm and do for 15 to 30 minutes twice a day.
Children under 15 years – do 10 to 15 minutes twice a day.
Not for pregnant women. Seriously ill people do it gently.
Anulom Vilom –
Close your right nostril with thumb and deep breath-in through left nostril
then – close left nostril with two fingers and breath-out through right nostril
then -keeping the left nostril closed deep breath-in through right nostril
then - close your right nostril with thumb and breath-out through left nostril.
This is one cycle of anulom vilom.
Repeat this cycle for 15 to 30 minutes twice a day.
Children under 15 years - do 10 to 15 minutes twice a day.
You can do this before breakfast/lunch/dinner or before bedtime or in bed.Remember to take deep long breaths into the lungs.You can do this while sitting on floor or chair or lying in bed.
Also everyday, press the centre point of the palm of both your hands 40 times with the thumb and press the tips of all fingers 40 times each.
To stimulate the pancreas to produce insulin: mandukasan - kneel down(with feet pointing inwards,and sit on the ankles/heels, Vajrasan position), breathe in and breathe out completely and hold your breath, pull the stomach in, press both your hands on stomach, bend forward as much as possible keeping the head straight, hold for 5 to 15 seconds and come back up while breathing in. Repeat this 3 times daily to stimulate the kidney and pancreas.Mandukasan2 - Repeat the whole process,but this time with with fists of both hands pressing against the stomach.Mandukasan can be done sitting on a chair, if you cannot bend the legs.
Continue the breathing exercises once a day, after the diabetes is in full control.