Hi :)
Welcome to the forum
but sorry to hear of your suffering and that you have to be here!....
I'm sure you find all the support and advice you need on this site to deal with the fissure. It's truly a great find for a fissure sufferer.
Yes, headaches and tiredness are side effects of Rectogesic but they don't affect everyone and you may be lucky. It's nothing to be scared of and definately worth a shot if it can get rid of your curse! I tried using it and it wasn't so much a headache but a "headrush" for me that just seemed to last, although it does calm down too. I found the effects stronger in the evening than the morning but did end up switching to Diliatizem in the end, which worked great for me and helped immensely in healing my fissure.
Please just give it a go. It did provide me with a lot of pain relief whilst initially using it and the pain relief and side effects still outweighed the pain of the fissure at the time. A lot of people use it very successfully.
If it's that bad, talk to your GP about switching to an alternative cream and hoping you'll find the relief you need very soon :)