For short-term relief from muscle tension I agree with others who recommend cannabis. (Unfortunately it is not prescribed for ME in the UK.
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I also get some relief from the sedating antihistamines diphenhydramine and chlorphenamine, but due to their sedating effects there are of course limits to when they can be used.
Quinine relieves muscle tension but is not safe to use often.
I also agree with those who say they have been helped by reducing grain intake. I get much less muscle tension since adopting a leaky-gut diet which avoids gluten, reduces grains and adds some supplements (I don't have dairy anyway), but it does take a few months to get the benefit, which is not just in muscle tension reduction but many other symptoms too. There is even a possibility that it can eventually cure the underlying pathology, or at least bring about remission.