DEAR DR. ROACH: I've been diagnosed with Type 2 diabetes and prescribed 1,500 mg of metformin. My A1C is now 6.0, although even when it was 5.8, my endocrinologist kept the same dosage. My primary care physician suggested I cut the dosage down to 1,000 mg, but my endocrinologist told me that taking 1,000 mg rather than the 1,500 mg is like "taking nothing." I've read there are advantages of taking metformin, so for now I'm staying the course. -- C.D.


ANSWER: Nearly all medicines have a dose response, meaning that more of a medicine generally means more effectiveness. However, they have toxicities as well, and again, the higher the dose, the greater the risk of toxicity. A prescriber's job is to find the dose with the optimum effectiveness at an acceptable toxicity.


With metformin, the usual dosing is 1,000 mg up to 2,550 mg per day. My experience is that 1,000-1,500 mg is nearly as effective as 2,000 or more. Many people will have stomach upset or diarrhea at high dosages. Using a slow-release formulation greatly reduces this effect.


As you identified, there are other advantages to metformin, especially helping people keep their weight down. It may be that the endocrinologist is concerned about this, because an A1C of 5.8% or even 6% is a lower goal than necessary for most Type 2 diabetics.


As a primary care doctor, I give a great deal of deference to the specialists who help manage my patients' medical conditions. It's possible your endocrinologist found that for you, the 1,000 mg dose wasn't effective but the 1,500 mg was, even though in most people the effect is similar.


DEAR DR. ROACH: I am a 55-year-old female who was recently hospitalized for a serious case of ocular shingles, although fortunately it did not invade the eyeball and my vision is intact. The shingles affected the left side of my face with severe migraines, eye pain, swelling, vomiting, fever and weakness. Should I receive the Shingrix vaccine, and in what time frame? I read that after having shingles, one should wait three years to receive the vaccine. I am otherwise healthy, with perfume/scent induced asthma.


I wanted to receive the vaccine last year at my annual wellness exam, but my primary care physician advised me to wait. She wanted to see what long-term effects the vaccine would have. I do not ever want to go through this again. -- A.S.


ANSWER: The new shingles vaccine, Shingrix, may be given as soon as the rash from shingles has crusted over, but the risk of developing a recurrence of shingles within a year of an outbreak is low. You can get it anytime within that year.


I understand that some doctors are reluctant to prescribe new treatments, but the data and years of experience with the Shingrix vaccine are convincing enough that I recommend it for everyone over 50 who does not have a medical reason not to take it. This is true for people who have had shingles before as well as those who have not.