I found the same but the health visitor today suggested the baby d drops instead as you don't have to give so much of it!
I use the colief vit d drops. 2 drops a day in his mouth
Syringe it into their mouth that’s what i did then give them the nipple quick 🙈
I pop it on my little finger and let her chomp on it
I also put a drop on my little finger and let him suck on it. Tried the nipple way and found it an unnecessary hassle
Just syringe into their mouth. Much easier than on nipple!