When he began drinking alcohol, Patrick would drink two beers after work to feel relaxed. Six months later, Patrick needs to drink a six-pack of beer to feel the same level of relaxation. This illustrates:

drug tolerance

Explanation:

Drug tolerance -

It refers to the reduced affect or reaction of any drug, because of consumption of the daily basis or over consumption, is referred to as drug tolerance.

Over use of any drug makes the body resistive to the effect of the drug.

The drugs can be alcohol, smoking, etc.

The drug tolerance gives the extent of drug use.

Hence, from the given scenario of the question,

The correct answer is drug tolerance.