Wow just seen this, poor Henry , what a worry!
I give Riley 1 and a half drontal tablet every three months, he has always been fine thankfully.
Its one tablet per 10kg weight, Riley weighs 13.5kg , last gave him it two weeks ago.
Drontal is all I have ever given Riley, at 6, 9 and now 12 months, will give him again in May.
Some on here are fine on drontal, some not.
Hope this gets sorted soon ! x