I wish I knew about contraindications but I am afraid I do not. I would think given that it is based in brandy, that anything that is contraindicated with alcohol should not be used. I have only ever used small amts and never heard if Rescue remedy until my vet told me about it. Until then, I used cocculine - which has been suggested by another vet for an older cat who was not a good traveller and would sometimes be ill in the car. Cocculine is a homeopathic substance that is found in health food stores or the pharmacy section of some grocery stores. Whether it works - as Dr. V has suggested I suspect very much depends on the animal. There are some independent studies of cocculine being done in France - where it is made - with good results. I would not give much credence to studies done by its manufacturer. (They cannot be unbiased.) I know of no studies offhand of Rescue remedy.
The scientist in me was VERY skeptical of these products - even when a vet I respect very much recommended them - but much to my amazement, both have worked for me with different cats. I have never tried them with dogs or bunnies! And I have only ever used a very small amount - with cocculine, when my younger Siamese was a kitten, I would give her half a cocculine tab before a trip - and she would sleep. (It seems to be a side effect in some animals but that is personal and anecdotal experience - and I loathe anecdotal experience and hate to provide it!).
When YY was older, I gave her a tablet and she was fine with that. I have used Rescue remedy when she seems to know we are going to the vet (I do not know why, she just knows, lol- and meows loudly as Siamese tend to do when they want to express their displeasure with something). When she was a tiny kitten (maybe 4-7 lbs), I would give her half the recommended dosage which was 5 drops and gave her maybe 2. I would either give it to her in her mouth or put some on her paw and she'd lick it off. When she was older - and she looks big in her pictures but she is still a svelte small Siamese - weighing about 11-12 lbs give or take a few ounces here and there. Beau - who weights less is also a meezer and I have not given him RR since he has a heart murmur and other medical issues and while it is likely safe, if it not necessary - he is quite laid back and never gets too upset about too much. He is not as demanding as YY - who is the alpha kitty, lol I have given it to my Sphynx cats who also are also both about 9-10 lbs. With fur, they might weigh more. (and I would gather if one discounts fur in other animals, you might get the actual weight!)
I do not use these products and I do use them only for my pets - frequently - they are very occasional use - and I am ironically not sold on homeopathic products and would never recommend them to my patients and comment on them only if asked. (Tho my own oncologist once recommended rooibus tea to help sleeping - it is South African and he has a theory that this is why SAfricans are so laid back - I don't know, lol). I am however allergic to almost any kind of tea so it did not work out.
Needless to say, neither product requires an Rx.
"There are two means of refuge from the miseries of life: music and cats" Albert Schweitzer