|Jeremy Brett as Sherlock Holmes|
On the left: his first appearance in "A Scandal in Bohemia"
On the right: in his last episode "The Cardboard Box"
From Wikipedia, here's the story of what was happening outside the box:
After the death of Joan Wilson, Brett struggled with filming the third Granada series, 'The Return of Sherlock Holmes' in late 1985. On the set it was noticed that his manic episodes, his excessive changes of mood, were getting worse and eventually grief and workload became too much; he had a breakdown, was hospitalised and diagnosed manic-depressive.
Brett was given lithium tablets to fight his manic depression. He knew that he would never be cured; he had to live with his condition, look for the signs of his disorder and then deal with it. He wanted to go back to work, to play Holmes again. The first episode to be produced after his discharge was a two-hour adaptation of "The Sign of the Four". From then on the difference in Brett's appearance slowly became more noticeable as the series developed. One of the side effects of the lithium tablets was fluid retention. Brett began to look and act differently. The drugs were slowing him down; he was putting on weight and retaining water. Brett also had heart troubles. His heart was twice the normal size, he would have difficulties breathing and would need an oxygen mask on the set. "But, darlings, the show must go on", was his only comment.
During the last decade of his life, Brett was treated in hospital several times for his mental illness, and his health and appearance visibly deteriorated by the time he completed the later episodes of the Sherlock Holmes series. During his later years, he discussed the illness candidly, encouraging people to recognise its symptoms and seek help.
That certainly answers the questions about Jeremy Brett. But within the reality of Toobworld, what was happening to the state of Holmes' health?
My intent is not to make light of the situation; instead I wish to provide a splainin for the changes to Holmes' appearance by thinking inside the box. Hopefully this would also be in keeping with the "Canon", since he is the televisual embodiment of the original stories.
Although we didn't see it happen on screen, Holmes had begun conducting his experiments into the longevity benefits from the royal jelly of bees long before his retirement to Sussex.
Many Sherlockians believe he was ultimately successful and was able to extend his own life-span (to the point where some claim that Holmes is still alive today!) As with any scientific experiment, there would be many trials and errors during research. And heedless of his own physical well-being (which we know he was wont to do with the use of cocaine and morphine when bored), Holmes would have had no qualms about using himself as the test subject in his royal jelly experiments. He may have been convinced that his physical stamina (as demonstrated in "The Speckled Band") was enough to withstand any detrimental effects caused by his experimental serums.
But each new test caused physical alterations to his body - the bloating, the softening of his once finely honed features.....
Based on what we saw in those last episodes, Holmes never gave a splainin, and Dr. Watson was probably too polite to enquire. And then again, it may have happened in a scene were not privy to.
For Jeremy Brett, that physical deterioration was permanent and he died not long after completing his final story as Holmes. But for Sherlock himself, life continued after the series finale. Eventually he hit upon the right combination of the serum and probably returned to his old self again... with the guarantee of a longer life to boot.