The drug in your drink is often the one you brought

Club drugs are psychoactive drugs that act on the central nervous system and
change mood, awareness and behaviour. They circulate mostly among young
adults in bars, concerts, nightclubs and at parties, and — like most drugs —
their nicknames shift over time and differ from one part of the country to
another, which is itself a reason nobody can be sure what they have been
handed.

Some of these are approved for particular medical uses. Every other use is
misuse.

The part that is commonly misunderstood

A date rape drug is not a specific chemical. It is any drug or alcohol
used to make a sexual assault easier. That definition covers two situations,
and most warnings only describe the first:

1. Someone puts something in your drink while you are not looking.
2. **You are already drinking, or already taking something, and a person makes
   it stronger without your knowing.**

Club drugs are sometimes used this way. So is alcohol, which is the most
common of all of them.

These drugs are very powerful. They can take effect quickly, and you may not
realise anything is wrong. How long the effect lasts depends on how much is
in your body and whether it is mixed with other drugs or alcohol — and
alcohol makes the effects of other drugs stronger, which can cause serious
health problems and death.

What actually reduces the risk

The advice is short, and it follows directly from the two situations above:

• Never leave your drink unattended. The glass you set down is the one
  that gets altered.
• Don't accept drinks from other people.
• If you are drinking from a can or bottle, open it yourself.
• Look out for your friends, and ask them to look out for you. This is the
  one that covers the second situation, because a person whose drink has been
  strengthened cannot reliably notice it themselves.

Source: MedlinePlus, U.S. National Library of Medicine.