However, estrogen bypassing the digestive tract and liver and entering through the skin is not converted to a new form before entering the bloodstream. Creams and gels applied to the skin also enter the blood directly and without modification but absorption of the gels, creams, and patches can vary from application to application, depending on the temperature and condition of the skin. Pharmaceutical compounding is frequently used to modify the dose, form, and additives of the preparations, based on instructions from a health-care practitioner. The primary difference between the popular promotions of CHRT and compounded BHT involves BHT advocates' use of regular testing of blood or saliva-bound hormone levels, use of individualized compounding rather than standard doses, dosing to attain certain levels in the body rather than for symptom relief, and the use of hormones for purposes other than relief of menopausal symptoms. Proponents of compounded BHT have been criticized by many mainstream medical sources for making unsubstantiated claims about its effectiveness for a variety of purposes, and for promoting it as more safe and "natural" than CHRT. Advocates for BHT have claimed that commonly compounded BHT preparations are not commercially available; this is false, as there are many FDA-approved hormone preparations containing bioidentical molecules available both as proprietary or generic brands. The exception is estriol, used in the compounded bioidentical preparations Triest and Biest—in 2008, the FDA banned estriol until a New Drug Application had been completed; these preparations are not approved by either the FDA, or Health Canada. Some advocates of compounding have also falsely claimed that customized compounding provides customized results; this is incorrect since it is aimed at producing a single hormone profile with absolute blood or saliva levels—which has not been demonstrated to be better than CHRT—and does not consider the rate at which individuals will differ in the activity, metabolism and excretion of the hormones. There have been no clinical trials directly comparing the effectiveness or efficacy of bioidentical versus nonbioidentical compounds. A 2010 article published in "The Medical Letter on Drugs and Therapeutics" concluded "There is no acceptable evidence that 'bioidentical' hormones are safe or effective. Patients should be discouraged from taking them." BHT is frequently associated with testing of saliva to establish a baseline hormone level and compounding of the substances by pharmacists (according to a doctor's advice) to produce preparations (and blood levels) of hormones that are individualized to the patients. There is no research demonstrating any benefit to either of these practices.