A chiropractor, whose treatment didn't help me) suggested that I might try 5HTP, available from health supplement stores (e.g. Healthspan online). I have to be careful not to combine it with sumatriptan which I usually take for my headaches but so far the results are promising. I take 100mg just after lunch and I've dramatically decreased the number of headaches I get. So often these improvements are short-lived (e.g. on prescribed medication) so I'll have to wait and see what happens, but I'll report back in a month or two.