As stated above, taking antihistamines with opiate drugs will significantly potentiate the opiates overall effects. This method is used often in medicine today as you're able to give a smaller amount of the actual opiate itself while yielding an increased opioid effect for the user. Antihistamines act as both a histamine antagonist and an anticholinergic(block the action of acetylcholine: a neurotransmitter involved in memory, learning, and blood vessel control:via parasympathetic inhibition)
Lilpsychotic where did you read this info that you're claiming? I myself have never read any material/studies that support that. Given an antihistamines anticholinergic effects(mainly parasympathetic system inhibition which is associated with 1st generation antihistamines such as benadryl)It can cause an increased heart rate and blood pressure. So i guess this may be your concern, however 2nd generation antihistamines such as (claritin or loratadine) have shown a greater affinity for the histamine type 1 receptors than for cholinergic receptors. This means less of a peripheral anticholinergic effect/parasympathetic inhibition effect. In other words less parasympathetic inhibition means the body can maintain a normal heart rate and blood pressure through the sympathetic/parasympathetic counter balance response. I've heard of antihistamines such as ketotifen(most commonly) used to upregulate beta-2-receptors that clenbuterol downregulates.
Here's a quick little chart demonstrating the nervous systems sympathetic/parasympathetic responses:
Sympathetic: Parasympathetic:
(Stress, Fight/Flight) (opposite response)
Heart Rate: Increased Decreased
Coronary Arteries: Dilate Constrict
Blood Pressure: Increase Decrease
Bronchioles: Dilate Constrict
Respiratory Secretions: Decrease Increase
Pupil: Dilate Constrict
Skin Blood Flow: Decrease Increase
Secretions: Decrease Increase
Sweating: Increase Decrease
Blood Glucose: Increase Decrease