Hi.. Welcome to HEALTHCARE MAGIC.. I have gone through your query and can understand your concerns.. As per your complain it seems that your teeth have got stained with tobacco stains and due to this there is discoloured teeth.. You should consult an Oral Physician and get evaluated and Scaling can be done to remove stains as home remedies cannot remove stains as they are tightly attached to teeth.. Secondly if you do not stop tobacco chewing there will be repeated staining of teeth and also can face other deleterious conditions like Precancerous lesions and even Oral cancer.. So it is better to stop chewing tobacco.. Nicotine Replacement Therapy like Nicotine patches, chewing gums etc can help.. For improvement in weight a balanced diet plan containing a complex blend of protein, carbohydrates and fats can help.. So consulting a Nutritionist and getting a diet plan formulated can help.. Along with it weight training or muscle gaining exercises under a physical trainer can help.. Back pain on left side can be due to causes like lower back strain, lumber injury, prolapsed disc, Sciatica, kidney related problems etc.. So consulting an Orthopedician and getting evaluated and proper x rays can help.. As of now take anti-inflammatory painkillers like Ibuprofen for relief.. Hope this helps.. Regards.. Lower back strain, lumber injury, Sciatica, prolapsed disc etc..