Do they work? Well, yes and no. I'll explain. First, I could find no propper randomised control trials. Just a Wikipedia page saying:
"Whilst there is considerable anecdotal evidence of Thiamine products being effective in the field (Australia, US and Canada), there has yet to be any clinical trials run to demonstrate the efficacy of these products."
Then I found this about Vitamin B over-dose:
"Thiamin (vitamin B1) is a water-soluble vitamin and as such, it is least likely to reach toxic levels. There is little danger of thiamin toxicity when it is taken orally. However, there is an exception. When thiamin is taken intravenously (injections), it has been reported to cause anaphylactic shock in few people. Symptoms of a thiamine overdose may include a feeling of warmth, weakness, sweating, nausea, restlessness, difficulty breathing, tightness of the throat, bluish colored skin, and death."
This weekend Liz and I tried the patches and neither of us went into anaphylactic shock.
They need two hours to become effective, so we put them on at 5pm when we landed, drove home, ate some food, then went out about 7pm to dunk the kayak gear and hang it to dry. We also messed about with our boats, sorting stuff for a forthcoming trip.
When we stood in part of the garden where there are very few midges, but where we would normally be bitten, they seemed to ignore us completely. So here, the patches worked.
When we moved to the most intensely midge area, around our Midge-eater machine, I found the beasts were more of a nuisance. I could feel them in my face and eyes and it was very irritating. The patches are not like a Harry Potter invisibility cloak. If there are lots of the beasts, they will find you. However, I did not get bitten, even here, and that was quite a surprise.
They weren't bothering Liz at all. "That's because I've put on two patches", she told me. "That way they'll go for you not me".
So that's the strategy. Wear patches but don't tell your friends. They get bitten, not you.
Incidentaly, the next morning Liz worked in the garden in exactly the same conditions and was badly bitten. This clearly isn't a full-on clinical trial, it's at best anecdotal evidence and based on just one evening's test.
But we will buy more patches.