Berbagai rempah daun memiliki efek positif ketika diuji secara in-vitro, pada hewan, dan uji klinis skala kecil,<ref>{{cite journal| author = Srinivasan K | year= 2005 | title = Spices as influencers of body metabolism: an overview of three decades of research | journal = Food Research International | volume = 38 | issue = 1 | pages = 77–86 | doi = 10.1016/j.foodres.2004.09.001 | id = | url = }}</ref> namun tidak jarang beberapa tumbuhan obat memiliki efek negatif.<ref name="ncbi.nlm.nih.gov">{{cite journal | authorlink = Pittler MH, Abbot NC, Harkness EF, Ernst E | title = Location bias in controlled clinical trials of complementary/alternative therapies | journal = International Journal of Epidemiology | volume = 53 | issue = 5 | pages = 485–489 | year = 2000 | doi =10.1016/S0895-4356(99)00220-6 | author =Pittler, M | pmid =10812320 | last2 =Abbot | first2 =NC | last3 =Harkness | first3 =EF | last4 =Ernst | first4 =E }}</ref>
Di tahun 2002, [[National InstituteInstitutes of Health]] mulai membiayai [[uji klinis]] terhadap efektivitas obat herbal.<ref>[http://health.nih.gov/result.asp/324 Herbal Medicine, NIH Institute and Center Resources], [[National Center for Complementary and Alternative Medicine]], [[National Institutes of Health]].</ref> Survey pada tahun 2010 terhadap 1000 jenis tumbuhan, 356 diantaranya telah memiliki hasil uji klinis mengenai manfaatnya secara farmakologi. Sekitar 12 persen dikatakan "tidak memiliki manfaat yang signifikan" meski telah tersedia di pasar.<ref>{{cite journal |author=Cravotto G, Boffa L, Genzini L, Garella D |title=Phytotherapeutics: an evaluation of the potential of 1000 plants |journal=J Clin Pharm Ther |volume=35 |issue=1 |pages=11–48 |date=February 2010 |pmid=20175810 |doi=10.1111/j.1365-2710.2009.01096.x |url=}}</ref> Dan berdasarkan [[Cancer Research UK]], tidak ada satupun pengobatan herbal yang terbukti secara klinis dapat mencegah atau mengobati [[kanker]].<ref name="cruk-herbs">{{cite web |publisher=[[Cancer Research UK]] |url=http://www.cancerresearchuk.org/cancer-help/about-cancer/treatment/complementary-alternative/therapies/herbal-medicine |title=Herbal medicine |accessdate=August 2013}}</ref> Berbagai pakar pengobatan herbal mengkritik studi ilmiah terhadap obat-obatan herbal karena tidak memasukkan pengetahuan historis yang dapat memberikan informasi mengenai dosis optimal, spesies yang detail, waktu pemanenan, dan target populasi penerima obat.<ref name=Fabricant2001/><ref>{{cite journal | author = Eric Yarnell, N.D., R.H., and Kathy Abascal, J.D| coauthors = | title = Dilemmas of Traditional Botanical Research | journal = HerbalGram. | volume = 55 | pages = 46–54 | year = 2002 }}</ref>
