You are not completely correct when you say that the Diazepam won't work for his "so called 'hip pain'" @Hatty65.
Amongst several other uses it is also used to relieve muscle spasms, which is because he it it rather good at relieving tired, and overworked muscles.
As well as working directly at relaxing muscles, Diazepam's most common usage is to help very stressed people to relax both physically and mentally. I am not an expert in how it works to relieve stress, I just know that it does.
So as an all-around relaxant, it can help a great many of the body's muscles to relax. Therefore it certainly could work in reducing hip pain. Also, when people suffer from pain their body usually tenses up, and the fear of something being about to cause themself to feel even more pain, indeed does make the body stiffen up with tension, and to reiterate, a tense body will lead to the sufferer feeling a significantly higher level of pain.
I think that without knowing the OP's partner, in person, it is quite unreasonable to judge whether they do actually suffer from a painful condition - hip pain in this particular case. Of course, taking anyone elses medication is both immoral and illegal, and on many occasions it could lead to a very bad medical outcome.
@Givenup2026, please be aware that if you tell your GP that your partner has stolen your medicine - for whatever reason - then your partner could be arrested. If the CPS then decides that he should be charged with the theft, there are several different things he could be charged with, including unlawfully taking drugs in order to sell them, in order to make a profit.
Apart from that, your GP could, but hopefully wouldn't, refuse to give you any more Diazepam as you have proved that you can't guarantee that they will be kept in a securely safe place, where both children, and thieves, could not get to them. I hope that your GP would be more understanding than that, and would give you another chance, but that is sadly not a guaranteed outcome.
I am not advising you to not tell your GP about your partner stealing your Diazepam OP, I just wanted to remind you that there are times when GPs have to break their patients' confidence, and the theft of your medicine might well be one of those occassions. I'm sorry Given up, but I don't feel able to advise you on your personal choices regarding your partner, as I have no actual knowledge of his and your relationship, and whether the good far outweighs the bad. But I don't believe that there is any couple who have a perfect relationship.