Hi, (Sorry for my bad English…)
Really appreciate if anyone can help …
Recently while I am playing I feel strong pains in the wrist
(left fretting hand).
pains mainly concentrate in the thumb and also in the lower part of the thumb, (picture).
Is there anything I can purchase to relief this pain?
I literally play for half an hour with awful pain until I can no longer hold the guitar.
Is there anything to do about it?
Thanks !!
First, see a doctor to rule out any serious repetetive stress injury. This sounds similar to one. Do not try and tough this out or play through it; take it seriously and see a doctor.
Second, see this thread:
I suspect you are gripping the neck much too hard.
I can only concur to what @howard said. Take a break and see a doctor. Even something as trivial as a tendonitis can become a chronic health issue if not taken care of.
I suspect you are gripping the neck too hard, causing undue stress on your thumb. Try relaxing your thumb and let your fretting fingers do the work. Your strap should be adjusted so that the weight of the bass is on your shoulder and you shouldn’t have to hold up the neck.
what Howard said. soreness, being uncomfortable is fine. never ever try to play through pain. give it time to fully heal, then begin again slowly. over time it should resolve, if not seek out medical help.
Really thanks to everyone for the answers !!
Really appreciate it.
What does @PamPurrs mean by “weight of the bass is on your shoulder and you shouldn’t have to hold up the neck.” ?
I just put a strap in a way that is comfortable for me - when i play at my work (office),
I do hold my neck a bit with my left hand so that the guitar does not touch the table
(the table is just below the neck of the guitar),
But it’s barely noticeable.
Most of the weight of the guitar is on my lap (sitting).
And the strap is pretty loose.
And I lean towards the guitar (not leaning on the back of the chair).
am i doing something wrong here?
If you let go of the neck, does it drop down? This is called neck dive, which causes you to have to grip the neck to hold it up. That’s what I meant.
Not sure if I’m understanding this correctly, but does “pretty loose” mean that your instrument isn’t really hanging off your neck and shoulder? Because in that case your strap would be too loose.
Have you tried playing while standing up? As @PamPurrs wrote, your body should support the weight of your bass, not your fretting arm and hand. Next thing you can try is fretting the strings without your thumb touching the neck. You may find that you need to apply a lot less force than you are used to. If fretting without using your thumb is difficult or even impossible, that could be an indication that your action is too high (the strings are too far away from the neck). A good setup should fix that.
First of all - I’m not really sure you understand how much I appreciate all the answers!
I did not know that this forum is such a Nice and cozy Place 
“If you let go of the neck, does it drop down?”
So Now I have come to work and checked this matter, and the answer is yes.
i always support the neck from falling down on table.
u think that this can cause the pain?
@Mike_NL
*" pretty loose” mean that your instrument isn’t really hanging off your neck and shoulder? *
Because in that case your strap would be too loose.
Yes the strap loose while I play sitting.
Okay, you might want to check out this video by our very own guru, Josh Fossgreen:
If you search YouTube for something like “bass strap height”, you’ll find plenty more.
Depends on how much you’re gripping the neck when you hold it up.
Have you tried standing while you practice? Do that and let go of the neck, see if it dives.
Okay,
So the update is - that the pain really subsided after I took all the advice from you,
I manage to play for an hour straight with really minor ( 
) pains, and it gets better with time,
So again - really thanks to everyone for the help !!