I always squeeze 2 lemon qaurters into a very large mug and throw in the skins as well (scrub first) then fill up with boiling water. Leaving the skins in gives more of a bitterness and it gives a stronger taste that can then be topped up with fresh boiling water when half empty. It’s nice when it goes cold too.
I tried a squirt of artificial lemon juice at my sister in laws house because whe didn’t have any fresh lemons… got a pounding headache so won’t be trying that again! Presume there was citric acid or some nasty in it? (This was very early on in my diet when I didn’t have much of an understanding… I wouldn’t try something as silly now!)