Why do antihistamines complicate glaucoma?

It is because antihistamines can have what are called "anticholinergic effects." They can sometimes block acetylcholine receptors, and not allow acetylcholine to function. In people with closed-angle glaucoma, this can lead to an increase in intraocular pressure (pressure inside the eye), which can worsen the glaucoma. That is why it is generally recommended that people with glaucoma do not use antihistamines. However, people with open-angle glaucoma (where the eye fluid can still move or drain) may be able to use antihistamines safely because they aren't as likely to have this increase in intraocular pressure due to the drug's effects. I would still ask the doctor before using the medication if you have any kind of glaucoma just to make sure it is safe, however.