I have a long established crown
It has started to give pain
The dentist says there may be a cracked tooth underneath
With an infection
He also says I would lose the tooth if he took off the crown
Is there anything I can take to deal with a possible infection?

I cannot tell you if there is something you could take for the infection... I'd expect your dentist to tell you so if there was.

I had a cracked tooth similar to this and I have to tell you it was better to have it removed, at least in my case. It was prone to infection and I risked getting disease in my jawbone. I suffered through that noise for way too long. Just get it taken care of.

So id talk to your dentist... even feel free to talk to a few... but if it is getting infected and they cannot treat it id have it removed. You can get a bridge put in and it looks fine, though it costs $$.

Hi, David,
I am 64 yrs old, and have been through just about every known crown, bridge, bad teeth, bad gums, and more fillings than an old asphalt highway!
I now have dentures, still getting use to them.
With that in mind, your Dentist would know a good antibotic to prescribe. These are only available with a prescription.
He/she can X-ray the tooth, and see if it is cracked.
I am not a Doctor, Dentist, but just a person with much experience. Sounds like you didn't have that much good tooth left, when the crown was put in place.
Yes, infection is the number one cause of an older crowned tooth starting to hurt... been there, done that.
If you have to have the tooth removed, you might consider a "bridge" of some type, to keep your other teeth from shifting; giving you "off bite".
I do wish you the very best, and good luck.

The costs might be different for you, but the tooth I had removed was about 3 from the back, top. So you couldn't really see it was gone unless I opened wide... but the cost of the bridge was somewhere around 2g.

Well that was the bad news. The good news was that I didn't need to get it replaced for a few years. The dentist said it would need to be replaced to prevent the shifting that fredg mentioned... but its not like that it had to be done immediately. This allowed us to set aside medical flex $$ through work when we could afford it.