Laboratory studies have shown that compounds found in Panax ginseng enhance the ability of phenylephrine to constrict blood vessels.¹ Controlled studies are necessary to determine whether taking Panax ginseng at the same time as phenylephrine will enhance the beneficial effects of the drug.

Antihistamines, including chlorpheniramine, can cause “anticholinergic” side effects such as dryness of mouth and heart palpitations. Henbane (Hyoscyamus niger) also has anticholinergic activity and side effects. Therefore, use of henbane with chlorpheniramine could increase the risk of anticholinergic side effects,² though apparently no interactions have yet been reported. Henbane should not be taken except by prescription from a physician trained in its use, as it is extremely toxic.

Many drugs used in the treatment of high blood pressure cause relaxation or dilation of blood vessels. Laboratory studies show that emodin, a compound in Polygonum multiflorum, also relaxes blood vessels. However, animal studies reveal that phenylephrine blocks the action of emodin.⁴ Controlled studies are needed to determine whether Polygonum multiflorum helps people with high blood pressure and whether phenylephrine blocks its beneficial effects.