Strangely the drugs that you take can affect your ability to meditate. I think sleeping drugs, relaxing drugs, anti depression drugs, affect you. But there may bar a host of other drugs that affect your ability to concentrate.
I think it also affects the ability to do yoga.
15.11.2018
I need to more about breathing meditation. I use the ticks of a piano metronome beats to synchronise breathing. The lung gets tired, and breathing get heavy, I thought it was ok, but found not. Breathing should be as silent as possible. Don't try to keep up with the beats, but count the beats. Think of the lung how it expands and contacts. When you are out is breath breathing becomes irregular, it's ok, keep meditating, after a while breathing becomes regular. When the body becomes tired even a little bit, the lung tried to get more oxygen, we should allow it, or we get more tired, and breathing becomes more heavy.