以上两种情形，有一种常见的保健品可能有帮助：美国花旗参。这话不是我说的，是美国国家卫生研究院（NIH）说的。NIH 旗下的国家补充与替代医学中心（National Center for Complementary and Integrative Health）官网综述了 Ginseng，下面又专门列出了 「American Ginseng」「美国花旗参」，发布在美国卫生部旗下 Medline Plus 官网，我编译如下。
Respiratory tract infections. Some research suggests that taking a specific American ginseng extract called CVT-E002 (Cold-FX, Afexa Life Sciences, Canada) 200-400 mg twice daily for 3-6 months during flu season might prevent cold or flu symptoms in adults between the ages of 18 and 65. People older than 65 seem to need a flu shot at month 2 along with this treatment in order to decrease their risk of getting the flu or colds. This extract also seems to help make symptoms milder and last a shorter length of time when infections do occur. Some evidence suggests that the extract might not reduce the chance of getting the first cold of a season, but it seems to reduce the risk of getting repeat colds in a season. However, it might not help prevent cold or flu-like symptoms in patients with weakened immune systems.
在 「Are there interactions with medications?」「花旗参与其他药物的反应」一节中，NIH 表示，若你在服用降低免疫力的药物，要慎用花旗参，因为花旗参会增强免疫力，从而使你服用的「降低免疫力」的药物失效。为什么会有人服用「降低免疫力」的药物呢？比如说，病人做了器官移植，身体的免疫系统会排斥「不明外来物体」，从而产生严重的并发症，所以他们需要暂时降低免疫系统的强度。
American ginseng can increase the immune system. Taking American ginseng along with some medications that decrease the immune system might decrease the effectiveness of these medications.
Some medications that decrease the immune system include azathioprine (Imuran), basiliximab (Simulect), cyclosporine (Neoral, Sandimmune), daclizumab (Zenapax), muromonab-CD3 (OKT3, Orthoclone OKT3), mycophenolate (CellCept), tacrolimus (FK506, Prograf), sirolimus (Rapamune), prednisone (Deltasone, Orasone), and other corticosteroids (glucocorticoids).