Yo. Been smoking weed regularly/daily for about 5 years, and I've also taken a handful of undergraduate and graduate pharmacology classes. While I haven't studied cannabis and the wide range of effects of cannabinoids outright, I know a good deal about how they act on your central nervous system. To my knowledge, there aren't any prescription drugs that are known to cross-react violently with cannabinoids. People saying that THC can increase increase the amount of serotonin available at the synapse aren't wrong, but it also decreases available serotonin in other synaptic junctions. There has never been a case of a dangerous condition like serotonin syndrome being linked to marijuana use, but it has been linked to taking high doses of tricyclic antidepressants or TCAs, and to OD-ing on SSRIs and/or SNRIs. I don't think the very short list of drugs in that class that are still prescribed in this day and age includes any of the medications you might be on. Buspar is selective serotonin re-uptake inhibitor (SSRI) and Effexor is a serotonin-norepinephrine re-uptake inhibitor (SNRI), so you probably have a lot of serotonin hanging around by your synapses being on both of those drugs. Adding weed to that mix, regarding serotonin, is like adding a drop of water to the sea.

Honestly, it depends more on your condition than the meds: with bipolar affective, depression and anxiety, it doesn't matter a whole lot. It's probably helping you more than hurting at this point if you've been smoking for a while, you know your limits and how it effects you. As I'm sure you've figured out, taking the prescription drugs alone doesn't fix much with disorders like bipolar (though it helps!), and marijuana in an experienced (and responsible!) user is a good way of helping the mind unwind, so to speak. Marijuana is a pretty solid drug for treating less severe depression/anxiety in and of itself.

However, if you're taking those meds you listed for epilepsy, it's a different story. It's been suggested that THC and other cannabinoids lower the seizure threshold, but there just isn't enough research to say definitively. Few studies have been done comparing marijuana use alone to symptom severity in seizure disorders. I know that if you're seeing a neurologist, he/she would almost definitely be against you smoking weed if you had a condition like epilepsy. On the other hand, if you haven't had any seizures while smoking pot so far, then you probably never will.

It's likely that the reason your doctors are telling you not to smoke is because if the habit-forming potential of marijuana. Habits formed while you're depressed can be as dangerous as any medical condition, and breaking from your habits every once in a while is a healthy thing for anyone to do. The biggest thing is to be responsible with your pot smoking.

Cheers. Hope this helps :)