Ingredients have to be listed in order of their proportions in the product, so if the first ingredient is the antibiotic, that's what there'll be more of.
When it comes to something like antibiotics, and the course is short, I tend to just get on with it. I'm a diet controlled T2, so don't have anything (aside from diet and exercise) with which to try to manage any resultant rise, but I tend to think if I need, as in your example, antibiotics, then ridding myself of the infection they have been prescribed fr is more important than a couple of days of erratic sugars, although the infection might be playing havoc with the blood glucose all by itself!.
Obviously, as a T1, the usual suggestion, as I understand it. would be to test plenty and adjust where necessary. If your numbers are very high mid-teens or higher, then testing for ketones is considered prudent.
You can also ask your doctor if they can prescribe the same antibiotic in tablet or capsule form. That will have a lot less sugars. Or see what happens when taking it and correct.
Sugar is not poisonous it is just not suitable in large quantities for diabetics. If this is small sachet, then even if it was all sugar it is not going to do any harm to have sachet or two and certainly better than having an acute infection.
According to on-line information, a 3g sachet contains 1.9g of sucrose. So it's approx 2 thirds sugar but it's only half a teaspoonful so probably not going to do you too much damage.