Is it against law to fire me for being on prescription medication?

You can be fired for no reason at all. California is an "at will" employment state. You can be terminated for wearing a blue shirt. [but if you PROVE with EVIDENCE you are fired for being gay, being born in another country, being female/male, practicing a religion or having a minor disability, you can demand reinstatement with back wages.] If the prescription drug is for a 'disability' and you can perform ALL 'essential job functions' DESPITE THAT DISABILITY you've got an Americans With Disabilities Act claim. Super, dooper HARD to prove, and very very very expensive to prove...so most lawyers can't afford to prove these for Subway and WalMart jobs.

That is a much more complex question that you likely could have assumed. There are some situations that would provide valid reasons why you could be fired for being on prescription drugs, and there are other instances where firing you for being on prescription drugs may have violated any number of laws. As they say, the devil is in the details, and without any it's impossible to answer your question. If you've been fired, you should set up a consultation with a local plaintiff's side employment attorney who can review your case and go over the facts with you.

Assuming your employer is covered, you possibly have an ADA claim if you have a disability or your employer regarded you as having a disability as interpreted under the Americans with Disabilities Act. If so, the issue will be whether you requested a reasonable accommodation which your employer failed to provide. If you were not capable of performing your job, even with an accommodation, then your employer should prevail.