I'm somewhat amazed and slightly disappointed by some of the comments here which amount to: "I know I am fine so I lie on the medical"....
The RSTC (or whichever form you are required to complete) is there to protect both the diver and the instructor and organisation. Some of the medical requirements are very serious - an epileptic fit or an asthma attack underwater would most likely result in a fatality. That is not to say that people who have suffered from these conditions cannot dive, and there are guidelines for physicians on how to diagnose potential problems that might be associated with diving.
New research by various bodies has allowed asthmatics and diabetics to dive whereas ten years ago it would have been an all-out "No". Some of the conditions, yes, may not be so serious, but they are listed on the form because under pressure, at depth, they may cause significant complications. Scar tissue from a major injury operation can increase a person's susceptibility to DCS because the blood flow in the scar is severly restricted. Some prescription medications may cause a change in blood pressure, some have a narcotic effect that might be heightened at depth. Ear or sinus surgery can create problems with equalisation.
Some examples: An intern at a place I used to work lied about his epilepsy. He had a fit on his Rescue Course and although it happened at the surface (thank goodness), he was medically classed as near drowning. Another guy (actually 3 separate cases) lied about their asthma and after being witness having an attack or taking their inhaler after stepping onto the dive boat were sent packing, not because of their asthma, but because they lied about it. Not long ago it took me 7 minutes to surface an AOW student from a safety stop because she didn't admit to having a deformed eustachian tube; she was in agony.
Falsifying information on a medical can, in the worst case, lead to fatalities. Whether right or wrong, this is probably going to result in families losing loved ones, and the instructor being dragged through the courts, potentially at vast personal expense, in what might well ruin careers and lives - it might be rare, but it happens.
If you have a medical condition and you are concerned about the fitness to dive medical, do everybody concerned a favour and get clearance from a physician, preferably trained in hyperbaric medicine. the Diver's Alert Network (DAN) can assist with locating one. I can't speak for other agencies but by PADI standards it is valid for a year, unless the condition changes.
Thank you to those who are honest, and to those of you who think you're fine and don't see the need for honestly disclosing your condition, please do not come within a country mile of my dive centre.
Thank you very much.
Safe diving,
C.