Hi, Since marijuana causes tachycardia which is a speeding up of the heart, it puts pressure on the heart. While your body has by now become tolerant to marijuana, if you take marijuana in excess amount of usual dose, you can feel the speeding up of the heart. The quality of the marijuana smoked and the amount smoked can both be responsible. In addition, since joints contain some tobacco as well, you are also exposed to the adverse effects of nicotine. Like marijuana, nicotine and the other harmful substances found in smoke are also bad for the heart. While your heart may have been fine at eighteen years of age, but continuously exposing it to damage by these substances for a long time can be very harmful. I suggest that you consider quitting smoking entirely. A few minutes pleasure is not worth the loss of a few hours from your expected life span, every time you smoke. Hope this helps you. Best wishes.