Drinking too much alcohol can weaken your immune system. A weaker immune system will have a harder time fighting off common infections (such as a cold), as well as AIDS-related infections. A weaker immune system also increases the chance that you will experience more side effects from your HIV medications. Smoking marijuana (pot) or any other drug irritates the lungs. You may be more likely to get serious lung infections, such as pneumonia.
Other common recreational drugs, such as cocaine or crystal methamphetamine (also known as "meth" or "speed" or "crank" and "Tina"), can leave your body dehydrated and exhausted, as well as lead to skin irritation. All of these things can make it easier for you to get infections.
The organ in your body that alcohol and other drugs affect most is your liver. The liver rounds up waste from chemicals that you put in your body. Those chemicals include recreational drugs as well as prescription drugs, such as your HIV medications. A weaker liver means less efficient "housekeeping" and, probably, a weaker you.
If you also have hepatitis C (or any other kind of hepatitis), your liver is already working very hard to fight the disease itself and deal with the strong drugs that you may be taking for your hepatitis treatment.
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