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. Though promoters of BHT claim that saliva testing can be used to "customize" the level of hormones for individuals, and tests are used to determine which hormones are supposed to be deficient and require supplementation, there is no scientific basis to support the use of saliva testing. Estrogens are secreted in pulses within and over days, resulting in varying saliva levels. Certain compounding formulations also attempt to use a single profile for all women, with no evidence that a specific profile is beneficial in all cases and no recognition that women differ in their sensitivity to hormones and metabolic rate. Testing-based customizing also does not account for much of the effects, and synthesis of hormones occurs within tissues rather than in the blood; therefore, blood or saliva levels of hormones may not necessarily reflect actual biological activity. Other concerns include lack of evidence that samples are stable during storage and transportation, poor replication of results and considerable variation among assays. There are also no studies that link symptoms with blood or saliva levels of hormones. The FDA recommends, instead, adjusting hormone therapy to the symptoms of the patient, and there is no reason to adjust the dosing or monitor patients receiving BHT. Skeptics of BHT have also pointed out that there is no certainty regarding hormone levels in the body. The North American Menopause Society has supported warnings about the potential harm BHT could cause, as it unnecessarily compounds drugs that are already FDA-approved in ways that lack an evidence base of safety or harm; this warning is supported by the Society of Obstetricians and Gynaecologists of Canada.