I hope that you can help me. From the outset, I will be open with you and admit that I do have OCD.

I was at a party earlier this week and everyone was drinking the same drink (and using straws). I was slightly intoxicated and when I returned from the bathroom I could not tell which drink was mine, and just picked up a random cup and began drinking from the straw.

It transpires that someone at this party is HIV positive.

After I left the party, I was walking home and a discarded cigarette was thrown from a balcony above. It caught my shirt and I rubbed the ash mark using saliva and my finger, oblivious to the prospect of germs. When I got home 10 mins later, I completely forgot to wash my hands and I masterbated (including touching the tip of my penis).

Am I at risk of HIV? Is testing advised?

Stopping to comment after seeing on the title you chose for your question, before reading the question itself:  HIV has never been known to be acquired by environmental exposure, even there is likely contamination with infected blood or body fluids. Unless there is a surprise when I read on, probably I will advise you there is no reason for concern.

Now I've read the rest. Guess what? There was no risk from either of the events you describe. HIV is rarely if ever transmitted by saliva; in fact, saliva is toxic to HIV and kills the virus. It is almost never transmitted by kissing (even prolonged, open mouth kissing) or by oral to penis (i.e. to the receiver of a BJ). And certainly the amount of saliva exposure from either a shared drinking glass or indirectly by contact with a cigaret smoked by an HIV infected person could not transmit HIV -- or any other STI.

So no worries and you definitely should not be tested on account of these experiences.

I hope this information is helpful. Let me know if anything isn't clear.

HHH, MD

2. Correct.

3. Also no risk. (The public toilet seat myth was debunked for STIs well over a hundred years ago.)