I'd be careful when taking Effexor (aka Venlafaxine) and using marijuana. It seems that some people have had very bad experiences while others are fine. You don't know which group you belong to yet.
A quick Google search shows me that a number people who have taken Effexor and shortly - even two hours after taking it - smoked marijuana have experienced a dangerously high heart rate and palpitations with some cases requiring medical help.
After taking Effexor it would be best to wait a number of hours before smoking cannabis and even then smoking a smaller amount than usual to see how your body reacts.
I have just started Concerta at the same 18mg and I feel that we are similar in our lifestyles (am meds with food, enhance the work day and end with some cannabis time)
I
I'm 17 and I've been on concerta for about 11 years, I would advise you to avoid taking adhd meds at all because they suck. But as for mixing them with pot, you're gonna feel a little more paranoid and just recently I've had a weird sensation within an hour of getting REALLY high. I would have heart attack symptoms, and no heart attack. It is awful and part of the reason why I'm going to get off of concerta. But I mean its different for everyone, personally I don't think it's a good time.