St. John’s wort has significant serotonin reuptake inhibition and can be rather effective so much so that when inadvertently taken with a SSRI can lead to serotonin syndrome. Problem lies with lack of regulation in supplement world so from bottle to bottle you could be getting wide fluctuations of bioavailability-again making potentially dangerous.
This is true, you should never take SJW with a prescription SSRI. But lack of government regulation doesn't mean supplements are unregulated. Private testing labs and self regulation of an industry can be as good or even better than government regulation. However it means the user has to take responsibility for finding out whether a particular product is what it claims to be.
First you vet the producer. A reputable company will have test reports available and the product will be as potent as it claims, in fact, they are often accurate to a closer degree than prescription medications are required to be. This might especially be true since prescriptions drugs have started to be sourced outside the USA.
Then one needs to educate oneself about herbal and supplement products. What's the difference between whole herbs and extracts? How do you read the shorthand to compare products and calculate equivalencies? And so on.
Then you need to get yourself thoroughly educated on the subject of your disorder, including all the theory and available evidence for the science behind how the supplement is supposed to treat it, as well as everything about standard prescription drug treatment. You need to be able to compare them. Not only do you need to compare the benefits of traditional prescription drug to alternative herbal or supplement treatments, you need to compare risks and side effects. Both have them so there are four categories you need to become thoroughly acquainted with: benefits of each and risks of each.
Not only do you need to know the equivalencies of various herbal preparations to each other, you need to know the equivalencies of them to approved prescription drugs. For example if you are on a prescription SSRI and wish to switch to an herb, you need to know how to begin substitution to accomplish a smooth transition.
In short, you need to own your own treatment, as opposed to blindly trusting a doctor. If you don't then you can waste money on worthless supplements at best or really hurt yourself at worst.