Horsetail (shavegrass) is used as a traditional herb for a variety of purposes.
But if you do a search for information about horsetail on the internet, nearly every monograph or article contains standard cautions that can make horsetail seem scary.
In a series of short blog posts, I want to examine each of these safety concerns to see what merit they might have.
In this first post, I want to look at the common caution that consuming horsetail is likely to result in a thiamine (vitamin B1) deficiency.
What Is the Basis of the Concern?
Almost all monographs and articles online about horsetail caution that consuming horsetail may result in a thiamine deficiency.
The logic goes like this: horsetail may contain a substance known as thiaminase. Thiaminase is an enzyme that breaks down thiamine.
There is research showing that in some animals – such as horses – if the animals eat enough thiaminase, they can end up with a thiamine deficiency.
For example, in horses, it’s been shown that if the hay contains 20 percent or more horsetail, the horse will develop thiamine deficiency symptoms within two to five weeks.
That’s a lot of horsetail, though.
In other animals – typically because of confinement and lack of access to a diversity of foods – such as some predatory fish, feeding large amounts of thiaminase-containing foods can also produce thiamine deficiency.
So it is not entirely unreasonable to be cautious when it comes to horsetail.
But it’s also important to put this into perspective. Horsetail is not unique in containing thiaminase.
If you want to be afraid of all things containing thiaminase, you should also avoid fish, crab, clams, blueberries, beets, Brussels sprouts, and red cabbage. 
But how many people are reported as having a thiamine deficiency from eating fish, crab, clams, blueberries, beets, Brussels sprouts, or red cabbage each year?
None that I know of.
Well, for one thing, thiaminases are commonly destroyed by heat. So cooking – as is commonly done with these foods – deactivates the thiaminase.
But even in the foods that one might eat raw – sashimi, raw blueberries, raw beets, etc. – the amounts of food eaten (and the thiaminase content of the entire diet) are too small to produce thiamine deficiency.
Okay. Okay. That’s all great. But what about horsetail?
Does Horsetail Cause a Thiamine Deficiency?
Theory is fine, but in practice, are there any reports of thiamine deficiency in humans associated with horsetail consumption?
None that I can find.
And if you find any credible case studies, please let me know.
The closest I can find is a single report that dubiously links horsetail use during pregnancy to autism symptoms in the child.  This case study is highly misleading because what it actually reports is that the mother consumed alcohol, restricted her diet in an attempt to lose weight, and had B vitamin deficiencies during pregnancy.
Oh, and she also took a weight loss supplement that contained, presumably among other things, horsetail.
We have no idea whether any of these factors caused the girl’s autism symptoms, but of all of the things, the miniscule amount of horsetail seems the least likely causative factor.
In any case, given the many, many people who consume horsetail regularly, if it really posed a threat of thiamine deficiency, I would expect to find case studies demonstrating a consistent connection between horsetail consumption and thiamine deficiency in humans.
Since I find none, I am highly doubtful that horsetail – especially in the modest amounts that most people are likely to consume relative to their overall diet – causes thiamine deficiency.
Furthermore, it appears that heat deactivates the thiaminase specifically in the case of horsetail.
In the abstract of a study I found on the subject, the researchers report that water extracts of horsetail contain little to no thiaminase activity. In contrast, the dried and fresh horsetail samples were described as having “potent anti-thiamine effect”. 
The water extract in the study was made using water at 100 C (212 F), which is boiling temperature at sea level.
In other words, it seems reasonable that a standard decoction or infusion in which the horsetail is subjected to high temperature water for an extended period of time will deactivate whatever thiaminase is contained in the horsetail.
I highly suspect that even raw horsetail in the very modest amounts found in most supplements is unlikely to be a problem based upon the lack of case studies linking horsetail to thiamine deficiency in humans.
However, it seems to me that a standard hot water extraction (infusion or decoction) from the dried horsetail herb is likely to be free of significant thiaminase activity.
I cannot guarantee that your horsetail hot water extract is free of thiaminase activity. But personally, I don’t worry about it.
I don’t worry about eating raw blueberries either, by the way. 😉