Interactions can be expected between cannabis and a wide range of drugs.  
Nortriptyline is a tricyclic antidepressant.  Cannabis interacts 
adversely with tricyclic andidepressants.  Cannabis, on its own, produces 
tachycardia as a side-effect.  Tricyclics do the same.  Combined, there 
is an additive effect, with a bigger increase in heart rate.  I have seen 
this with cannabis + doxepin (heart rate >150, as well as BP 99/75, 
dizziness and severe anxiety).  In this case, doxepin's hypotensive 
(blood pressure-lowering) effect (another common side-effect of 
tricyclics) was also potentiated.  Tricyclic antidepressants have a 
certain effect on the heart.  This effect can be described as 
cardiotoxicity.  In normal dosage, in individuals with no heart disorder, 
this causes no problems at all.  (In overdosage, tricyclics can produce 
serious cardiac arrhythmias.)  I suspect that in the interaction with 
cannabis, the cardiotoxic effect of tricyclics is somewhat potentiated.  
(I don't know if this has produced hazardous changes in ECG rhythm.)  The 
effects of cannabis + nortriptyline in particular have been documented.

I remember seeing a couple of references to all of this in Martindale's 
"Extra Pharmacopoeia", 29th edition, under 'Cannabis', but I haven't read 
them.