There's not a lot known about how HIV drugs interact with illicit or recreational drugs, though this is changing. However, if you do take recreational drugs, there are some common cautions you could follow:

• Avoid taking HIV drugs and other drugs at exactly the same time: wait at least a couple of hours between doses.
• Ritonavir, and possibly other protease inhibitors, may cause dangerous, even fatal interactions with ecstasy, methamphetamine and other amphetamines.
• Drink plenty of water.
• Start with a smaller amount of any illicit drug and monitor any unusual responses.
• Seek emergency medical help if you experience dizziness, sudden drowsiness, blurred vision, heart palpitations, vomiting or any other severe or unexpected effect.

(This information from HIV Tests and Treatments, AFAO: 2000)

For more detailed info:

• Download the AFAO pamphlet Dangerous Liaisons: interactions between anti-HIV and recreational (party) drugs (AFAO 2002). (PDF 606 KB). You will need Adobe Acrobat Reader to view this document.
• Check out the AFAO Parties site for more information on illicit drugs.