I'm not sure where you get those figures from Dave because each tablet of Septra DS contains 80 mg trimethoprim and 400 mg sulfamethoxazole, and the inactive ingredients docusate sodium (0.4 mg per tablet), FD&C Red No. 40, magnesium stearate, povidone, and sodium starch glycolate.

Is there another antibiotic that you're taking as well?

The 'normal' dose for Septra is as you describe... one tablet twice daily. Hopefully things will improve for you soon and I'm sure that mrsD will be able to give you more information when she returns but if there's anything that's particularly worrying you, then I'd be happy to try and find some information for you.