How many ml of codeine (1mg per drop) in 500ml of drink?

Can’t be said. I see specific dose recommendations critical. Dose data, as you find online, represent averages from the experiences of many users. What amount an individual needs to feel the desired effect or from what amount of adverse effects there are, can be said to a person who is not.

Although in some substances one knows very well what quantities these usually act, one never really knows how these will affect the individual. People can react very differently to drugs. Some better, some worse. Some can even react very sensitively genetically.

In the case of too high doses or in the case of people who convert the active substance to morphine particularly rapidly due to genetic conditions, symptoms of opiate poisoning can arise. These include euphoria or increased drowsiness, a declining respiratory drive (temdepression), blood pressure drop, disturbances of arbitrary movements (ataxia) and muscle cramps.

→https://www.netdoktor.de/medikamenten/codein/

If you want to take this kind of thing, you have to get to the effect individually. In doubt, especially with the first experiences, so rather “too low” dose instead of “too high”.

General information about the typical effects and side effects as well as the risks associated with consumption can be found here:

• https://www.gelbe-liste.de/active substances/codein_785
• https://drugscouts.de/lexikon/purple-drank
• https://psychonautwiki.org/wiki/Codeine