Hmmm, not sure, but I personally would advoid taking any kind of strong medication at the start of labour. If you end up needing further medication it could interfere with dosages... what ever you take make sure you tell any health professional who supports you in labour. Say you needed to have an emergency anaesthesia this would be very important.

Non medical pain relief, i have found, works well because it doesn't leave you feeling drowsy. Some strong medications will probably make you want to sleep/lie down... whereas an active labour is usually the best way to go (generally speaking). For me hot water does wonders for pain relief. I'd try that first. A fit ball under a hot shower can really hit the spot once labour is established.