Antihistamines have side effects that can include drowsiness or restlessness.
Drowsiness is tough to conquer. Venerable Moggallāna struggled with that a lot even though it was not from antihistamines. Perhaps the longest sutta on drowsiness is AN7.61, which has a list of many things to try:
AN7.61:2.3: “So, Moggallāna, don’t focus on or cultivate the perception that you were meditating on when you fell drowsy.
Discuss your medication with a doctor. Treatments very tremendously between individuals. For example, my allergies were gone after I became a vegetarian. And meditation itself helped:
MN62:5.3: “Rāhula, develop mindfulness of breathing. When mindfulness of breathing is developed and cultivated it’s very fruitful and beneficial.”
In general, whatever you experience, the Buddha's advice to Rahula may help detaching:
MN62:3.2: “Rāhula, you should truly see any kind of form at all—past, future, or present; internal or external; coarse or fine; inferior or superior; far or near: all form—with right understanding: ‘This is not mine, I am not this, this is not my self.’”