Ranitidine is a syrup so you give it by syringe. Ask for a 1ml syringe as they are the easiest to use. It's dosed by weight and the weight range for under 6 months is 1-3mg per kg x 2 per day, sometimes they will prescribe x3 per day. For over 6 months it's 2-4mg per kg but some Drs will go to this for younger babies. The syrup is usually 150mg per kg so the dose will be very small which is why you need the 1ml syringe.
It tastes disgusting - it's bitter mint, but babies get used to it really quickly. We found the best way was to lie them on the floor swaddled (or just arms swaddled to keep them out of the way!) and then get the syringe into the mouth as far to one side as possible. You will need to get it in in tiny bits to start so she doesn't get enough in her mouth to spit back out. Getting the syringe to one side and as far in as you can helps to bypass the taste buds. After a couple of weeks both of mine were willingly sucking down
It can take a couple of weeks to work and they tend to start at the lowest dose so it may be that she needs more before it works rather than it not working. It's very weight sensitive so she will need regular reviews as she grows. Both of mine needed the full 4mg per kg for it to be effective. Ranitidine starts to deteriorate after it's been open for 4 weeks, so keep an eye on how long you have had it as you may notice symptoms returning.
You will probably still need the thickener or a reflux formula as all the ranitidine does it reduce the acid, it doesn't stop the reflux action. Ranitidine is a histamine antagonist, when you chew, you release histamine which triggers the acid cells in the stomach to produce acid. Ranitidine reduces the amount of histamine the body produces so it reduces the amount of acid produced, it doesn't stop it a PPI drug like omeprazole will do that.
My health visitor told me it was ok to add a bit extra water to formula if they were constipated - not a lot, but for example if you are making a 5oz bottle you fill the bottle to just over the 5oz mark and then add the amount of powder for a 5oz bottle. That little bit extra in each bottle can help. Check with your Dr or HV though!
HTH!
Laura