As some people have said above, Feminax is just paracetamol and codeine (co-codamol). If this works for you do yourself a favour a try an own brand (Boots or Superdrug) co-codamol - it's far cheaper!!
I used to use Feminax all the time as it used to also contain an anti-spasmodic (hyoscine) which is the main thing that will relieve cramps and severe pain (trust me, I know - I ended up in hospital because of it at one point - the docs thought it was a urine infection, sent me home with drugs that made me sick, then discovered it was in fact 'just' period pain). Feminax then stopped working for me and I discovered they'd removed the magic ingredient. Fortunately there is a solution...
If you are really at your wits end with period pain, try using co-codamol, alongside Buscopan IBS Relief. Buscopan is an anti-spasmodic and the co-codamol is the pain relief. Together they work wonders. I have been using them for about 2 years now - it is perfectly safe to take them together (I checked with my pharmacist first) and as long as I take them I'm fine and can get on with life as normal. If I don't take them, you can bet that I'll be doubled up in pain unable to do anything!